NOTE: I know nothing about SQLServer.

Anything eating 400 MB on a 512 MB box running Windows is probably going to cause much disk thrashing. Windows likes to keep a big hunk of RAM free to make loading new programs seem fast. (E.g. this 768 MB box running Win2k, Thunderbird (18 MB), Mozilla (94 MB), BOINC (10 MB), NAV (NOPDB=50 MB), Acrobat Reader (35 MB), Copernic Desktop Search (33 MB), and a bunch of the usual utilities has 294 MB RAM free and 84 MB paged kernel memory). My guess is that it's throwing stuff it needs out to the pagefile and thrashing itself.

More RAM would probably help a lot.

Rebooting it would probably help, at least until the SQL process eats 400 MB again.

My vote is for rebooting it (once everything is put away properly). If it just started acting this way, I'd probably make sure the virus and spyware protection was up-to-date too... :-(

Luck!

Cheers,
Scott.