It could be a memory problem. Your new RAM could be incompatible with your old ones or the motherboard. Sometimes it helps if you simply put it in a different slot (I think; it's been a while since I've had to deal with anything like this).

Step one: make sure the RAM is properly seated in the slots. Step two: make sure that everything else is plugged in securely. Step three: pull out the new RAM, then see if the problem persists.

I just had the same problem, wiggling the memory in and out of the slots fixed it.

Also, sometimes windows has to be rebooted twice when you add memory. On the first failed try it notes some info for the second attempt.

Um, steps 1 1/2 and 2 1/2 and 3 1/2 are to reboot and see if anything changes. You probably figured that, though.

I removed all the old ram. BIOS can see the ram, and I confirmed with the mfg that it should be compatible.
I'll try to put the old back in tonight and see what happens...

Odd thing, though.
it registers as MORE than 4gb in bios. 4.5 or something, iirc. my system max is 4, so maybe that?
(oh, and I rebooted a few times.. not counting when windows did the rebooting on its own.)

You know, I have had a similar problem, and the reason seemed to just be "bad RAM", as in the new RAM I had just purchased was corrupted coming out of the box. It sucked because I did all this stuff, including install a new hard drive and power supply, when the answer was staring me right in the face - bad freaking RAM.

Is the 4.5 gb displayed as 4096 MB? This is 4GB in binary multipliers: one GB is not 1000 x 1000 x 1000 Bytes, no matter what the Hard Drive manufacturers tell you - it is 1024 x 1024 x 1024 Bytes (normally shown as 1024 MB), as computer memory is counted in binary "thousands" (1024 bytes).
I would guess that you don't have the right sort of memory installed -- many third-party RAM suppliers have little clue about what memory to install, whatever they tell you. I have frequently had the wrong sort of RAM suggested -- you need to check your Motherboard manual to see what sort of RAM is compatible.
Try two things:
1. Put the old RAM back in. Does the problem go away? If not, you have probably disturbed something else while installing the RAM. Make sure all the cables are pushed in tight and the other boards are in tight.
2. If the old RAM works, try swapping this out for TWO of your new RAM sticks. If the computer works with this, it could be that the PC just does not like having so much RAM. If not, you probably have the wrong sort of RAM. There are many different sorts: single sided vs. double-sided (SIMM vs DIMM), with or without parity, and lots of different speeds. Although some are compatible (e.g. 100 MHz can be replaced by 133 MHz RAM), some are not (you cannot replace a SIMM with a DIMM and expect it to work).
If the new RAM is your problem, I would check your Motherboard manual or Google your PC make and model if you have lost the manual that came with the PC.

I actually did check the mfg website, I SHOULD be able to run 4gb as four 1gb modules (aren't all 1gb modules 2 sided?).
however, as I said in the ask.metafilter post... there were two systems with my number. I found out when my roommate tried to get me a video card for xmas. The site said I had pcie. I have just pci.

OK.
Old ram is in and I'm chugging along as per usual.
I tried to install a pair of the old and a pair of the new, and it did boot, and windows was fine for a bit, but then programs started crashing and the whole thing shut down.
Two of the new ram did nothing.

so I am thinking it may be a compatibility issue.
bah.