Bob,
You've posted very little to go on (when does it lock up, doing what, is it
consistent, time related, process related, what services are running, what's
the OS (Home or Pro), is SP-1 installed, is it networked or not, cable
modem, dial-up, etc... etc...)
A good percentage of the time, intermittent lock-ups are caused more by
video drivers than any other device. So in order for you to start narrowing
this down, you need to minimize the system in order to eliminate potential
conflicts or buggy drivers / firmware.
I assume that you have checked your hardware device list and everything is
okay and that your system is virus free too. Turn off and AV software and
run for awhile, if it still fails leave it off during the rest of the tests.
1. Reduce your video attributes and run for awhile - any difference?
YES > Check for updated driver(s) and verify video card is XP compatible.
ATI and nVida make great cards and both have driver bugs that can wreak
havoc with a system..
NO > Change video driver to generic video driver and run with that. If it
still fails, re-install video drivers and go to next step.
2. Remove any USB devices and reboot and run a test period. Do you have the
latest drivers for any USB devices?
3. Remove and parallel port (LPT1) and serial port devices. Do you have the
latest drivers for any printers or serial port devices (modems, X10
equipment, printers, plotters, etc.)
4. Verify the mouse / keyboard software is latest version. Try changing
either/both of those devices to see if it makes any difference.
5. MS Scheduler, MS auto-update features, spyware, McAfee, Symantec AV's,
firewalls, etc. - turn them all off. Be sure to disable the clock updater.
If you're on a dial-up and your system is trying to do an update every (x
minutes) and it tries to connect, that may be the freeze you're experiencing
while the system waits for timeouts (can take upwards of 10 minutes
depending on your configuration.
6. Since you mentioned it is a custom system, is the correct memory being
used or did they go cheap on that ? Is the system over-clocked ? Check the
BIOS (if you know how) to verify settings for CPU voltage, bus speeds,
device settings.
7. How big is the hard drive ? If over 137Gb, is 48bit addressing enabled
and have you checked for any BIOS updates that may be needed? Chipset
updates may be available also so check with the vendor's site.
8. BIOS Updates may also be available (as well as new drivers) for your
DVD's, CD-R, CD-R/W, and may need to be flashed.
As you can see, you can go down many roads in troubleshooting this and we
could end up writing volumes on telling you what to do. But seeing as how
my answer is longer than the info you provided - it's your turn now.....
Bob S.