from the wonderful said:
ok!! How do I know what to delete? Because normally those are files
belonging to the operating system or programs!
Tell us where the space went, and we'll tell you what looks
unreasonable.
Hiberfile.sys, for instance, is as big as the amount of RAM you have,
and you only need it if you use hibernation. System restore points and
content indices can result in a large 'system volume information' folder
... reduce system restore size, turn off content indexing.
Similarly temporary internet files (stored in your user profile) will
consume however much space you let them (although on a new system the
amount should be small).
Pagefile.sys may have gotten set to 1.5x your RAM size. That's probably
too much if you have 1GB of RAM, and are not a power user (although
hopefully the actual minimum file size is rather smaller).
Your turn to provide some more information - without knowing where the
space went, we can only guess at what the solution might be.