Well, in Vista's Disk Cleanup, everything is safe to delete... Download Program files, Temporary files, Temporary Internet Files, Recycle Bin...pretty much everything..
If you are unsure though about what files would be gone, especially in recycle bin, the safest thing to do to clean space is check only the Temporary Internet Files and anything else with Temporary in it....
Tip: If you want to clean up even more space, go to More Options in Disk Cleanup and at System Restore and Shadow Copies click" Clean up". This will delete the old Restore Points and keep only the latest one....Saves a lot of space, provided you won't need to restore your system to points before the last one.