Huge problems with my pc

Last night my pc starting crashing constantly. Only this time, it wasnt the usual nv4_disp problem. I got several PAGE_FAULT_IN_NONPAGED_AREA and a few others. Once that happened, I could barely keep the machine up for 5 minutes at a time.

After working on it today, I now have win98 on my C: partition, XP home on my D partition, and my E partition with my useful (but not really critical) data.

XP crashed while trying to install it several times, and had problems copying files from the CD. Once i got win98 installed on C (no problems here), i ran the winXP install from there, and put it on D, which seems to be doing ok.

Currently, both win98 and XP are only recognizing 384MB of ram. Odd, considering I have 2 sticks of 256MB in slots 1 and 2. Does this mean one of the DIMM's died? I think that would explain the page fault errors.

Now that I think about it, booting from the winXP CD and installing from scratch had problems, but it did ok when i installed from the win98 partition.

If it sounds like a bad memory stick, how do I find out? I only have 1 machine that the DIMM's will fit in. Just swap them one at a time and see if one shows up as only 128MB? I just flashed my bios today, could that have caused a problem? I'm pretty sure win98 registered only 384MB before I flashed the bios though (isnt it limited to that much anyway?)

Another thing... for some reason, winXP home will not recognize my modem from a clean install. Its a US robotics 56k modem, just bought it last november. I had the same problem with 2 other brand new modems, it just won't install the drivers for it that are, according to USR, built into winXP. The only way I've gotten it to be recognized is to install win98 first, install the drivers there, and then install xp over the top of it. This way it can connect to windows update and get new drivers. Is there any way i can save these drivers on my 3rd partition so i can find them later? I've only been able to get them through windows update so far.

And one last thing: can I have 2 installs of winXP if they're on different partitions? I installed xp on my D partition because that was my only choice, other than going over the top of win98 again. If i can't save my modem drivers, can i try installing a fresh copy of XP on the C partition, and then format the D?

Sorry if this doesnt make any sense, let me know if something needs clarification. Here's my system info:

Win98, Original oem
WinXP home oem

Athlon XP 1600+ @ 1400 (default)
Gigabyte 7vtxe, BIOS version F6
2x256MB Crucial DDR2100 @ 133
IBM 75gxp 40gb drive: 16GB C:, 16GB D:, 8 GB E:, all fat32
Plextor 16/10/40 CDRW
Leadtek winfast gf3 ti200, nvidia 23.11 drivers, default clocked
Netgear fa311 10/100 PCI
USR 56k faxmodem PCI
SB audigy