On a 32 bit XP computer you just cannot use the whole 4GB of physical RAM. It'll always be capped at 3GB.
What happens is that the 32bit processor's use 32 pointers to address the bytes in the RAM. So there are 2 raised to the power 32 (2^32) possible addresses. But, in addition to RAM a good number of these addresses are also used to address other memories like the BIOS, the video memory etc. This uses up one fourth of the 2^32 addresses previously mentioned. So you're left with only 2^30 addresses for the RAM.
These many addresses can address no more than 3GB of RAM (2^30 = 3GB).
This is the reason you're only seeing 3GB of installed physical memory. It has got nothing to do with whether you've a shared video memory or not. This limit will exist in ALL 32bit computers.
In windows XP there is a way to go around this limit. It's called Physical Address Extension (PAE). This can take you up to 4GB (but not beyond). I'm not sure how to enable it. Try googling for it.