Critical Problem.

Hello all, I would really appreciate any help.
Computer: Dell Dimension 8400 Pentium4 with Hyper Threading.
Factors of problems are:
- Going from XP Home to XP pro; Blue screen of death
A. Blue Screen: BAD_POOL_CALLER, physical memory dump complete, and ***STOP: 0x000000c2 (0x00000007, 0x00000c4, 0x00340045, 0xE1F0FFFC)
- I have replaced stock mem 2x512mb (was told was first problem) with a 1 gig ddr2 stick, I have two of the 1gig sticks but the same person told me to trouble shoot with 1 stick at a time in each of the four slots. Well, no configuration of the sticks or slots (haha) will keep it from crashing.
- By crashing I mean, that no matter what mode I tell it to start in, the computer will always revert back to regular mode and pick up where it left off during the upgrade. it takes about 3 or 4 minutes (acting like its working), the minutes left to completion even changes from 39 to 34. At this point of the upgrade the status is "installing drivers".
- Please understand I never see the desktop. I see the xp loading screen with scrolling colored bar,as usual. BUT THEN it loads directly to windows XP upgrade screen. **I hear the disk spinning rapidly, then a "clack" sound, then silence, and THAT is when the screen flashes black, but then goes back to the upgrade screen for about 15-30 seconds. After that it flashes back to the "blue screen of death!"
-The fun starts all over again.

Okay so,
1. Do I need a new motherboard?
2. If so, do i need a dell replacement, or will another work?
3. If another one will who can tell me (if you can PLEASE do) which aftermarket board WILL fit/work?
4. Can I do something different other than replace the motherboard?