You are hitting up against the 4 GB total addressable memory limitation
inherent in the 32-bit version of Vista/XP. While the 32-bit versions
of Vista/XP can address a total of 4 GB of memory, device drivers need
to use some of that address space for memory mapping. Device drivers
start at the "top" of the 4 GB address space and work their way down.
Click here for more info: 'The system memory that is reported in the
System Information dialog box in Windows Vista is less than you expect
if 4 GB of RAM is installed'
(
http://support.microsoft.com/kb/929605/en-us)
You may be able to change a setting in your computer's BIOS to allow
you to successfully install with 4 GB installed but 32-bit Windows will
only "see" and use about 3.12 GB of the installed memory.
The short answer is to either remove 1 GB of RAM or install a 64-bit
version of Vista. I went with the latter choice myself.
Hope this helps.