Hi, i recently finished building my first comp. I used my old hard drive, and installed XP over it, so I could still keep the majority of my files with my new build.
 
My problem is, whenever I try to start up CoD2 or CoH: opposing fronts, my computer crashes and goes to the blue screen. These 2 games are the only 2 retail games I have tried. 3dMark also crashes my system in the same manor.
 
I often play an open source, free RTS game which uses OpenGL, which runs perfectly fine.
 
I feel this may be a result from installing XP over my old XP, leaving some drivers behind.
 
My specs:
e6750
en8800gt
IP35 pro mobo
2gb corsair twinx ram
corsair 520hx PSU
 
Does anyone have any suggestions as to what I can do?

Did you install the drivers for your new hardware when you re-installed XP?  I doubt your harddrive is causing a BSOD when it boots fine... more likely sound or video drivers...
 
Is it possible this is a DX9/10 issue?

I installed all of the new drivers after I re-installed XP. Im thinking it is a DX9 issue, because it only happens when I run directx games.

What does the video card manufacturor say about DX9 compatability?

I've only found sites which say it has DX10 support and OpenGL 2.0. I haven't found anything about incompatibilities, so I'll keep searching.

I assume the DX tests all run successfully with no problems?

Yes, all of them ran fine.

1) Did you install the latest DX9 files from Microsoft?  I think the newest are November 2007 release (maybe even newer...)
2) Monitor your PSU voltages and make sure they are within ~5% of nominal.
3) Have you run Memtest86? (at least for ~8hrs)
4) Can you run Prime95? (at least 12hrs)

It could be unstable do to RAM errors and RAM timings. Do what PCKid777 suggested and also try removing all but one stick of RAM, because it could be one stick gone bad. I had a hardware failure lastnight, one of my Crucial dimms died while the other one was fine. Whenever I tried to play COD 4 it blue screened me every few minutes crashing.

I am very doubtful it is my ram. It only occurs when starting up DX games. I say this as in, it happens instantly when I try to load up the game. (Put in cd, start screen comes up, press play game, crash).
 
The open source game I play does require quite a bit of ram, and runs perfectly fine, on the largest maps, deep into the game, with many units performing actions. All of the monitoring programs I have installed all detect my RAM fine, and when I run multiple things at once, (game, music, PS, Illustrator, Virus Scan), it will show me using over 1gb of ram from the sticks.

When you have strange things happening, often the best way to troubleshoot is by elimination.  Even if it's doubtful, you be sure by simply removing one stick of RAM and starting a game.  Do this once for each Mem stick, same results means your RAM is probably fine.
 
What kind of error are you getting?  Does it produce a memory dump?  Do you have your system set to dump memory on BSOD?  Errors in syslogs?  Devices in device manager that have any problems/conficts?

Yes, my system does dump memory on BSOD.

Use windebug to analyze the dump, it might give you a clue as to which piece of hardware is generating the exception.

Highly doubt it's the RAM, I wouldn't even bother.  It's a GPU issue.  Try re-installing direct X if you haven't already.  If that doesn't do it, make sure you have enough power going to the card.  If that doesn't do it, RMA the useless piece of junk.

If you're getting a BSOD I would suggest referencing this place:
 
http://aumha.org/a/stop.htm
 
I too, doubt it's RAM.  
 
What concerns me is you didn't format and reinstall, you did a dirty install. I specifically avoid that because I've run into problems left and right with it. Also check to see if there is an event log created. You might be able to get a better clue as to what is going on.  
 
Right Click 'My Computer' => Manage => Event Viewer => System if I remember correctly. Look for the X's and the !'s. Sometimes they will carry some more details with them. You never know, you might get lucky. All in all I'd definately say software glitch somewhere, hardware sounds to be okay.

 
 
Specifically, what is the BSOD?
 
0x000000_ _ _?
 
usually just referenced as 0X07B for inaccessible boot device. when not notated it's 0x0000007B.

 
 
Wow I didn't notice this when I first posted but installing over an existing install is not very smart, never heard of somebody doing this before. Simply back-up all your info/data to a DVD or flash drive then after you format and install Windows XP you can still install your older files and pictures. Installing over another install is like throwing new carpet over dirty carpet, the stains are still there and will present more problems later.

Well, I'd hafta say something highly unexpected happened. I tried to install  dx 9 again from windows, same thing happened. Out of almost boredom I clicked install direct x from my call of duty 2 cd. It said DX has been installed the second I clicked it. Now everything works perfectly...
 
Im thinking that directx from my old xp install was corrupted.

 
 
You mean for now things are ok, but later on I wouldn't be suprised if more problems came up. Why you would want to do a dirty install is suprising to say the least.  

well, what's done is done... try also keeping all drivers updated... inc the nvidia ones... can't remember what they're called

 
True, but whats done can be redone and done the right way. If I had known he had done a dirty install I wouldn't have even bothered to help him. People create their own problems by cutting corners, then come here for help and wonder why this is happenning to them. STUPID