I'm interested in upgrading my CPU and Mobo (maybe RAM too) but working off my existing windows installation.  
 
Will this work if I basically take my HD out of its existing environment and drop it into a mobo with new hardware?  
 
I'm using vista 64 bit and am hoping windows is smart enough to boot and recognize its new surroundings...

If you are moving to a new MB with the same chipset, a 'repair' install will work. A VISTA 32 repair install worked for me going from same Mfg. MB 965P to a new Q965 chipset.  Your HD has the old MB system/chipset drivers, so your best chance would be to try a repair install.  My suggestion would be to reformat the old HD and do a fresh install of the OS, then load the new MB system drivers.  I wouldn't even consider doing a repair install again otherwise.  Reformatting and reloading the OS is too easy.

Just to agree with badge, you may also find that it will simply run but ask you for a lot of drivers once you're in windows (the usual graphics, sound etc).
 
I have also found that different Intel chipsets seem to do this trick quite nicely so you're not limited to the same chipset (hopefully).
 
One word of warning, Windows XP will want to reactivate.  If you have an OEM sticker you'll probably have to go through the process, fail it, change the key so that it reflects the actual OEM sticker key.  Then answer a bunch of automated questions over the phone about why you're reactivating (a bunch of stupid questions like are you installing this on a second machine...).
 
Just be sure to select/say that your motherboard died, it worked for me when repairing a family PC.

My 965 -> P35 upgrade wouldn't boot Windows, in safe mode it failed on (or after) crcdisk.sys.

It may work.  You'll have to try it and see.  I usually find that you have to reinstall to get a functional stable windows.  Be prepared for an install.  Is it because you don't have the CD that you don't want to install?  

So long as you have the Product key, it doesn't matter what disk you use to install, I used a disk from work last time, but put in my original product key and had no problems.

I have always done repair installs. I have not done a complete new install of Windows in over 4 years and moved the same drives through at least a half a dozen complete new builds. I never have any trouble. And I am picky about the way my PC performs and acts....it better be quick, smooth, and no errors what so ever.
 
I did however choose to start fresh and did do a clean install when I upgraded to Vista and I confess I have not tried to move Vista yet. However the day is coming soon, it's about time to put the old AMD rig out to pasture and sadly move to a high end something that has "Intel Inside"  (shudder   )
 
But,,,, there are a lot of people who do have problems, and swear they can never get it work correctly.
 
My advice, back all your important stuff up to another drive. Try the repair install, you have nothing to lose. If it doesn't work out for you, just start over fresh.

 
Reformatting and reloading the OS is easy yes, but I always hated the reconfiguration and reinstallation of apps.  
 
And I think I'm moving to a new chipset too... I have AMD/Nvidia now, and I'm looking to move to an Intel chip.

 
No I have my trusty $280 DVDs of Vista 64 ;-]  I'm probably going to have to do the full reinstall... as much as I don't want to... I think I'll at least try to do the HDD swap upgrade first and keep my fingers crossed... it sounds like some others have had success.

 
 
I am very much in the same boat with regard to AMD.  I was all w00t when the x2 dual cores hit the market and built an awesome system.  But with my current config I've experienced random reboots in XP Pro and now to a lesser extent in Vista 64.  
 
I've worked on some solid intel machines at my last job and my current job and just really like the stability I feel on an intel chip.  Hate to say it but this would be my first ever intel build...

I'd just back up the info and reinstall... easier. although the "repair" function (if it works as well as it did on windows ME might be possible) i would still just back up the info and do a full install... I don't see any reason not to if you back up the info... saves time and headache to just do it right.... mobo/cpu/memory replacement is basically an entirely new pc, with new drivers... and your new hardware is going to be trying to run off of drivers that are for the old hardware... meaning it's not going to run properly... so do yourself a favor, back up your info, and reformat... and if you're selling/giving a pc away... write the drive to zeros beforehand understanding the risk of course.... oh and make sure  you're ready to get microsoft support on the line because shista drm is going to require that you only install any given disc of shista on only ONE pc... meaning when you installed your new mobo/ram/cpu, they'll likely force you to buy a new copy of shista. If that's the case www.ubuntu.com that might come in handy.  
 
and as for the fellow above me... it's not your proc causing the problems... it's prolly an I/O or I/B error more likely if you know what i mean. :-p
 
k, bye now.

If you go take a windows class at any community college or university - you will gain access to MSDN Academic licensing = free XP and Vista.  At the very least being a student will land you a copy of vista for 60 bucks.  
 
On a side note, i just swapped from an X2 4000+ MSI system, to a Q6600  and Asus P35 board plus different hard drive.   Complete reinstall of Vista Ultimate and no activation issues.  

Just a thought...
What if you tried installing drivers for the new hardware before you switch?
My line of thinking is that if Vista knows how to use the new hardware before the switch, maybe everything will go smother.
Or maybe I am just thinking crazy....
In any case, make sure to back up any important files before you make the plunge.

 
 
badge is correct, whenever changing chipsets or mobo's its best to reformat and start over. Five months ago my P5B Deluxe (P965 chipset) died on me and I kept my OS, then someone told me that my older P965 chipset drivers are still stored on my HDD. I am glad I did a reformat,  I ran many benchmarks before reformating and after I was all done my benchmark scores (FPS) were much higher than before.