Ok. I'm very computer literate, but this one has me stumped. Read this one all the way through as the first suggestions you might suggest have likely already been tried. OS is Windows XP Home Edition SP2 with all the latest updates, Hardware is AMD Athlon XP 1800+, 1.5GHz, 2.00GB RAM.
System randomly begins grinding on the 500GB hard drive, partition #2 (the G: Drive). During this time, the hard drive activity light blinks rapidly at first, then steady on as system begins to grind to a halt - even the mouse becomes very sluggish. CPU at this point is pegged at 100%. Symptoms are brought on by anything - opening My Computer, starting Media Player, burning a disk, playing a game, etc. System can be stable for 20 minutes, then grind to a halt for 10 minutes, then mysteriously CPU goes back to 2% (or less) and hard drive grinding activity comes to halt. Upon rebooting, system can be unresponsive at the XP login screen for up to 2 minutes before I can click the user name and login. After logging in (ten minutes later), everything is quiet and the PC is usable for some time before the grinding begins againn and the CPU pegs at 100% (no time pattern that I can see).
What I've Tried:
During the hard drive grinding, Windows Task Manager shows 100% CPU, but the the CPU offending process is not listed (show processes from all users is checked). The sum total of all the processes CPU usage including Task Manager equal 3-5% yet the status bar shows 99%-100% CPU usage. I/O activity shows normal with a few bytes read/written every now and then. Downloaded AnVir Task Manager....again, no offending task is shown. Halted every non-critical task/process, still 100% CPU. Turned of Search Index service and halted every non-critical service. Loaded latest BIOS and reset settings to default. Adjusted "Paging File" to many different drives/sizes/settings/system managed, etc. Reinstalled drivers for IDE drives and tried 3 different graphics driver sets (current, 1 generation old, 2 generation old). Removed ALL cards except AGP graphics card, unplugged CD-ROM & DVD ROM & Floppy, unplugged all USB devices, uninstalled all USB driver hubs. Ran ChkDsk /f & defragged. I've scanned system with 4 different virus scanners & 5 different MalWare detectors. All clean.
I've disabled EVERY startup program using MSConfig. Prayed, kick the desktop, tried the 3 foot drop test (kidding) and everything else I could think of. No matter what I try, every now and then 100% CPU and total hard drive grinding for 10-15 minutes before I can use the system again. In safe mode, I do not see the 100% CPU.
Anything else I can try? I don't really want to reformat because it would be 4 days of work to get the computer back to the configuration I have it in.