gigabytes of hidden disk hog under winxp

--restore points.
By winxp default, system restore points use up to 12% of C: drive. For a 100G disk, about 12G space can go missing -- select all top-level C:* objects and Alt-Enter to see total size (A) and compare with C: drive usage (C) by Alt-Enter.

If you are like me you don't need that much restore data. Don't bother to delete "all restore points except the most recent". Just reduce 12% to the minimum 200M. You should see (A - C) going down to about 200MB. See http://www.techrepublic.com/article/10-things-you-should-know-about-windows-xps-system-restore-tool/6048545.

You are even allowed to turn off system restore completely, though not recommended by anyone.

Note the C:* alt-enter counts recycler and page files -- not as hidden as the restore points.

--recycler. I put recycler on D: drive
--"temporary internet files" folder is virtualized. You can move the target folder. http://support.microsoft.com/kb/301057

--------now the visible disk hogs
--i386? http://ask-leo.com/can_i_move_or_delete_my_i386_directory_to_free_up_some_space.html
--C:\MSOCACH can be deleted. This folder is the temp directory for the MS Office installation files. If you have selected to run all from my computer option next to all the components of MS Office then you already have everything installed in the Program Files and will never need this folder....EVER
--Windows.edb, produced by the WindowsSearch indexer.
Can be relocated via control panel -} indexing -} advanced
--page files
--hibernate
--temp files....

------- compress old files?
Compressing ProgramFiles, I saved about 1 GB.
Compressing C:\WINDOWS\Microsoft.NET, I saved about 0.5 GB
http://serverfault.com/questions/1788/on-disk-cleanup-do-you-compress-old-files-to-save-disk-space says Uncompress won't hurt performance too much.