Win32 build problem in latest nightlies

The following is happening:

I can only run the latest nightlies once.  After I quit them and restart 
them, they freeze on the splash screen.  If I start up an older build 
(say, RC1) and select a different theme, quit it, and then start the 
latest nightly again, it will run again.  But yet again, only once. 
Rebooting the computer has no effect on it.

My system is as follows:
Celeron 433
Windows 98
256MB SDRAM
20GB HD

Is this a mozilla bug, or a problem with my machine?  It seems awful 
suspicious, and a little frightening so close to the release of 1.0. 
I'm afraid I have no programming ability whatsoever, so the best I can 
do is to act as the messenger.  Hope this gets fixed soon!

Any ideas?

PTR