You may have some .zip files that were uncompressed there as part of a download that you may want to keep, but other than that, yeah empty c:\temp if you want.
Go find an app called CCleaner from Piriform.com. It'll take care of this stuff for you.
**Oh yeah, go find your \Application Data folder too (in c:\Documents and Settings). There may be crash dumps in there, and they can be huge.