Sorry, I could not help it: according to this article at OSNews, a non-official streamlined & updated distribution of Windows XP boots with 50 MB of RAM consumed.
Compare that to the whopping 550 MB needed to boot Vista and you will wonder why oh why oh why oh why...

This being said, if you are really in the need to regain your computer and you just can't throw back Vista because of drivers' availability, hardware compatibility, whatever, you could try seamlessly installing the latest version of Ubuntu on your machine with the help of Wubi.
I just did that recently, and as a result, my computer boots now with a min of 300 MB RAM consumed (out of 1 GB). The system runs snappy (even though it should be slower than a plain vanilla Ubuntu installation, i.e. with a dedicated partition on your disk) and I can do true multitasking again!
Just browsing the internet while listening to some music is again a relaxing experience (my computer just went nuts writing this post on Vista and with 4 everyday applications open).
Also, I'm rather pleased with the polished look of the latest Ubuntu version, and I can still go back to Vista with no effort (just reboot), whenever I really have to.