Plague of BSOD on older XP box

I'm just looking to see if anyone can suggest avenues that I haven't
followed, short of full stack debugging and such. My client has 6 year-old
Dimension towers. When I first approached the problem box, it would blue
screen within minutes of being put under any load, generally with 8E or 50
codes. Ran utilities to clean up reported problems with Redirector Buffer
Manager and misc. registry issues and got a highly suspect Dell graphics
driver out of the mix by replacing the onboard adapter with a PCI model.

The above resulted in a dramatically more stable -- days, now, rather than
fractions of an hour under load -- system, but one still plagued by BSOD.
The codes, however, have changed. The consistent error is now 4E (99, 0, 0,
0), with the kernel apparently implicated.

Not interested in heavy stack debugging here, since my advice to the client
from day one has been to replace the hardware and bypass the problem (pure
economics). Client also runs WinPoint financial software and a hosting-site
web-app that downloads a number of plug-ins that are beyond my control. Note
that client has a number of similar systems, with only this one experiencing
the problems.

Any ideas? I'm about out.

Thanks.

JDEverett