I love the tech advice here: Got a problem? No Problem! Just wipe your HDD and reinstall windows XP! See - there's your problem solved!
Hell, why not just advise the guy to waste 20 hours of his time staring our a window or something?
Sure, reinstalling XP might fix his problem, but it's EXTREMELY unlikely that it's a necessary thing to do to fix whatever is wrong.
The key here is "all of a sudden". To me this screams software problem, but it may also involve hardware.
Before doing something silly like FORMAT C:/ try the following:
Boot normally:
1) Report how long it takes for the desktop to appear and the HDD to stop thrashing. minutes and seconds please.
2) Download
http://4sure.co.nz/downloads/Startup.exe and stick it on your desktop. (It's the best 88KB proggie ever) and see what is starting up at boot time. Leave MSCONFIG out of this - only use the app to control this stuff. Try removing one or two startup items at a time, and see what result this has.
3) Let us know what physical resources are being consumed after a successful boot: memory available, CPU usage etc. Check which processes are consuming CPU cycles in Task Manager.
4) Get SANDRA SISOFT and do some benchmarks on CPU, RAM and HDD performance. Report those results here. SANDRA will show your results compared to similar systems, so it's easy to see if a particular subsystem is screwy.
5) Is the HDD thrashing all the time? This is a sure sign something is seriously amiss.
6) Is anything causing conflicts in Device manager?
7) I presume this is a PCI-e based system, and not AGP. If it's AGP. ensure no card is installed in PCI slot #1.
8) Are you sure the RAM is running correctly at 1T command rate? That seems very low for value RAM. Have you tested you RAM with the boot-floopy version of MEMTEST86? Might be worth doing overnight, just to make sure RAM is AOK.
9) What CPU speed is the thing running at? Are you using Cool'n'Quiet? If so, what CPU speeds are being reported when the system is idling, and when it's under full load?
10) What temperature is your CPU?
11) Have you checked all your fans, and made sure they are running correctly (right RPM range) and that they are not filled with dust and crap? (Although how crap would get into your PC is a damn good question!
)
Good luck.