Well, that narrows my ideas down to just about none unfortunately. If it were a driver issue then you should have been going through this from the start. There aren't many hardware or software issues that I know of which run on a three week clock. And if there have been no hardware or software changes then there also shouldn't have been any performance changes. If it were hardware then reinstalling the OS shouldn't fix the problem. Vista's registry tends to be pretty durable, and I can't imagine what could have happened which would have caused this. I can take a few more shots in the dark.
-Try running checkdisk (My Computer>(right click on the Raptor) Properties>Tools>Error-Checking)
-Narrow down the possible culprits, pull the 8800 and see what happens, (that board has integrated I assume?), do the same with your backup hard disk
-The trial version of Sisoft SANDRA has a whole suite of hardware diagnostics utilities.
http://www.download.com/SiSoftware-Sandra/3000-2086_4-10556571.html?tag=lst-1
-How well does she boot into safe mode? That would at least help us narrow down which software components can or cannot be causing the problem.
-Check your task scheduler ('task' in the start search), see if there's anything there which kicks in after a few weeks. I don't know of such a task but we may as well cover the bases.
-Run a virus/spyware sweep from safe mode using updated versions of: your Avast install, AVG Anti-Rootkit, Spybot, Defender and any other scanners you have access to. (probably not the problem and really time consuming so it's your call)
-Check the reliability and performance monitor ('perf' in start search), click on reliability monitor and see if you can find any changes (installs/uninstalls/windows updates) which occurred just prior to the problem.
I wish I could be more help, but I've been fixing Vista machines at work since it came out, and this is a first. But then I have yet to work on an SP1 version (also, are you running Vista 64 or 32?). I usually have a pretty good feel for these things, but only when I have the system in front of me. My best advice would be to keep reducing the possible causes.
Best of luck.