"RAM: 512 mb stock/2.5gb w/ upgrade"

2.5GB? Are you using two 1GB sticks and one 512MB stick or something? To my knowledge RAM has to be installed in equal portions, either filling all your slots or only using enough to get to 512MB, 1GB, 2GB, etc.

Installing it any other way could result in either the motherboards' failure to detect the RAM, or damage.

 

By upgrade to you mean you replaced the old RAM, or added to it ?

Go back to the old memory and troubleshoot from there first. You may have more than one problem, the new RAM and "something else".

 

OK, after fooling around with it some more, I've gotten it to boot with the XP setup disk in the optical drive. But it won't boot without the CD in the drive.

After the POST, I get "C: boot from CD," then "Press any key to boot from CD." I ignore it, and after about 5-10 seconds it boots normally from the hard drive.

And it's recognizing all the RAM: two 1 gig sticks and a single 512 mb stick.

okay.

1) There are questions pending.
2) You may be have two separate problems. Failing to establish that the system works normally in it's previous condition is a bad idea, wastes your time and (most importantly) wastes the time of people trying to help you. There are plenty of other people asking for help who may be more cooperative.
3) It's a bad idea to run mismatched memory until you can prove that the "matched" sticks of 2 Gybte RAM work well.
4) Are the 2 sticks of 1 Gbyte the same ?
5) What are the make & model #'2 of the 3 different sticks, and what are their individual timings & voltages ?
6) How are their timings recognized in BIOS ?
7) What voltages are they getting (displayed in BIOS).
8) Burn a disk with memtest (link in my sig), and boot to it to test your memory in various configurations.

 

OK, after fooling around with it some more, I've gotten it to boot with the XP setup disk in the optical drive. But it won't boot without the CD in the drive.

After the POST, I get "C: boot from CD," then "Press any key to boot from CD." I ignore it, and after about 5-10 seconds it boots normally from the hard drive.

And it's recognizing all the RAM: two 1 gig sticks and a single 512 mb stick.

 

You can try this route:

Boot to your Windows XP disk. Press "R" to get into Recovery Console. Type "fixboot" and press "Enter". Type "fixmbr"

and press "Enter", type "bootcfg/rebuild" and press "Enter". Eject the disk.


Reboot.