Um, if control-alt-delete does nothing in windows xp, that says to me that the cpu can no longer process instructions.
A bad drive or bad memory would at least trigger a blue screen with a parity error, invalid instruction, etc. Bad drives also usually make ugly noises which end up being pretty obvious even to a non-technical user, but that's not always the case.
Enough bs'ing though, you want to isolate the problem instead of guessing what it might be and then decide what (if anything) to replace and if it makes sense for you to do so, or if all you need is a system restore or complete reinstall, etc.
For kicks, you can run scandisk:
http://www.duxcw.com/faq/win/xp/scandisk.htm But I'm going to assume that the scan ends up clean.
The easiest way to take the HD out of the loop and stress test the cpu/memory/motherboard is to burn a copy of memtest
http://www.memtest86.com/ onto a cd or floppy and then start up the computer by booting to that (I would be happy to mail a CD if you don't have a burner, or have a geeky friend near you do it for you, the .iso file is all he/she would need if their computer has a cd burner and burning software.) Let memtest run for 24 or 48 hours or until the system freezes again if you can afford the downtime. When it freezes, immediately reboot and check the cpu temp in the bios (del or F1 or F2, watch the post screen and it should tell you). If you missed the freeze, just restart memtest until it happens again. Acceptable ranges for P4's northwood cores are under 60 celcius, 65 isn't completely melted, but 70+ is asking for trouble. Prescott cores handle heat better. Still, most P4's with proper case and cpu cooling run under 50 with load or into the low 40's when idle. Now, watch the cpu temp in the bios drop over time as the cpu is now idle so it should be dropping. Note the idle temp. If it's dropping from 70 when it freezes to under 50 when idle, you have identified your problem. If it's 55 or cooler immediately after freezing, then you have ruled out heat as the culprit, at least for the cpu.
If it's not heat and scandisk is fine, and memtest reports no errors, the next checks I'd do would be on the power supply. One way to do that would be with motherboard monitor, if it supports your system:
http://www.snapfiles.com/get/mbm.html motherboard monitor is a very comprehensive tool, but if it doesn't list your exact model, don't trust any of the numbers it reports.
Memtest will report any memory errors, so if you are able to make it 24 or 48 hours with no memory errors (and no freezing), you have validated the physical integrity of your memory and the motherboard. If you do see memory errors, don't immediately replace the memory, try removing it and reinstalling (i.e. reseating) and also try using different slots (though if it's a dual channel setup, make sure you use 2 slots with similarly-colored tabs for each matched pair). If you play around and can't get the memory errors to stop, then it might be time to consider new memory.
Assuming all of that is fine and dandy, I'd probably give up, yank the drive, sell off the cpu, memory, and case + power supply + motherboard in 3 separate ebay auctions, and buy a new Dell 3000 or 4700:
http://www1.us.dell.com/...4&l=en&s=bsd We actually had this exact problem (machine freezing up completely, requiring a hard reboot by unplugging from the wall, even the power button wouldn't restart it) with our dev db server at work, I wasted hours trying to diagnose it. The solution ended up being that we needed the retail cpu fans that should have come with the processors, and it only took one call to the case manufacturer to figure that out. Our IT guy likes to build his stuff up on his own, and he used the heatsinks that came in the cpu box, but either we didn't buy retail cpu's or we got shorted on the heatsink fans. Anyway, $40 (2x $20) bought us one year of completely flawless service so far (though our new server room is like 1/2 the temp of the old one.) A new cpu heatsink fan is $20 or so, well worth the $ if you can identify that as your problem. Add an extra $8 for thermal paste if the cpu fan doesn't come with that.
Hopefully some of this was more valuable than completely useless garbage. I really wouldn't start messing with the OS until you've verified that the hardware is 100% functional. If the thing freezes to the point where it won't take a control-alt-delete, that says to me you have hardware problems to sort out.