Off the top of my head, three possible options, provided with no guarantees and little experience of Vista:
 Trim any extraneous running services from services.msc and msconfig. Accessed via the Run command.
 Find and uninstall any software that is both active in memory AND extraneous, such as graphics card utilities and system tray junk.
 Use a program that can control and release RAM addresses once they are lo longer used.
They all work on XP - but like I said, no guarantees. I thought Vista had improved memory address release, but there you go...