Just a quick question here, if I have windows xp installed on my HD, then I buy all the EXACT same components agian and put the original hardrive in, will everything (installed applications, windows xp itself) work?
If you have the EXACT same hardware it will work just fine. But depending on the age of the hardware it could be difficult to find the same stuff.
Even if you don't have the exact hardware you can still get it to work. You just have to redo some drivers and sometimes work out some quirky problems. Did that to a Win98 machine back in the day when the house got hit by lighting and a bunch of electronic stuff got fried. Doesn't work as well as a clean install, but does save you from having to install everything again.
The only issue you might encounter is with MS themselves. They do keep a count of how many times you activate a license, and eventually will cut you off. Your parts will have different IDs, even though they are the same models, and so you will have to activate again.... probably. If you have to DL something at some point requiring you to "validate" then you definitely will.
I'm not saying that it's a big issue. I'm just pointing out that you are also spending one of your activations.