I read on Tim Sneath's blog that some Dell's have a certain chipset that may cause issues if you have over 1GB of RAM.  I don't know if this fits your case but here's the quote from his web site.

"A number of Dell and other laptops have Broadcom 440x integrated network cards. The Windows XP driver for this card works on Windows Vista, but it will cause a blue screen crash on machines with more than 1GB memory. To resolve this problem, edit the boot.ini file to add a /maxmem=1024 switch for the Windows Vista partition. This will reduce the available memory for this partition to 1GB. Alternatively, you can of course disable the network driver."

Hope this helps.