Cannot delete many files in my Program Files, I keep getting these errors: "you need permission to perform this action."
Or: "....dll is in use!"

What are you trying to accomplish ?

Holy crap! Don't go in there deleting stuff. The program files directories aren't as disconnected with your system as they were 20 years ago. You shouldn't go in there unless you explicitly know what you're doing. Otherwise you could cause MAJOR system instabiltity that could result in having to do a soft rebuild of your system. You don't want to do that. It is costly in time and effort.

If you want to remove a program then you need to do it through your control panel and use the programs (win 7) or add/remove programs (win xp). Control. Use that to remove stuff from your computer.

Sometimes after an uninstall, the program leaves its directory behind (usually empty of data and just a folder structure). You can try right clicking the folder and try to take ownership. I think it only works if your logged in as administrator though (win7 and vista at least).