How do you mean clean? If you want to just clean up temp and old files you can d/load Cleanup 4.52 from stevengould.org and run in conjuction with windows disk cleanup. But there are a number of other programs available. CCleaner is another program which seems to get a good rap, I personally haven't used it.
If you want it in the "like new" condition, you will have to format and reinstall the OS, because once you start using it, stuff gets added.
If you're concerned about removing any traces of files you've deleted but keeping the ones you still have active, look for a diskwipe program that will clear the free space on the disk. Use this after you've done disk cleanup, emptied recycle bin, cleared internet history, compacted your email.... In the best case, get a program that will also overwrite the slack space at the end of files - lots of old data can be recovered there by skilled forensics analysts.
And just which three letter agency are you hiding from - FBI, CIA, FGF* ?
Is a little hard to advise on exactly how to clean as:
You have not advised what OS you are running (not ALL cleaner apps will run on all versions of Windows)
What exactly you are trying to scrub (ie, deleted personal files, browsing history etc)
Are you attempting to restore to factory settings after a malware infection?
Are you trying to scrub for re-sale? (unlikely given response to re-format suggestions, but have to ask)
Has your system suddenly become a shared console?
If you are trying to set up a system that will not keep a track record of what you are up to, or keep a record of sensitive data, this is something which needs to be set-up right after Windows is installed, not way down the track. Honestly, depending on how long this user account has been running, it may well be quicker to back-up important files and start again.
The other option is to set-up another user-account (must be Admin if will end up being the only user-account), transfer files from your current account to new account (might need to set said file locations to "shared" in order to have privileges), and then delete old account. Will then need to run a 3rd party disk-cleanser to scrub free disk-space, and a reg-repair tool (one built into CCleaner should do) to repair any registry entries damaged by the switch over.
Of course whether or not this advice is dependent on what you are actually trying to achieve.