Therema's right up to a point. When you uninstall a programme using the add/remove in Control Panel, sometime you get a message saying that Windows cannot remove ALL the files. This implies that there are some bits of the programme still littering the registry and system. Whilst it's usually OK to leave them, it's niggling to know that they're still around.
After a normal uninstall I do a search for any files that refer to the programme I'm trying to delete, even 'hidden files'. There are always a few that refer to it. When found, I send them to the Recycle Bin. I leave them there until I've rebooted a couple of times. If the PC is operating OK, I feel confident enough to delete them from the recycle bin.
Occasionally I get a message that I cannot delete a file because another programme is using it. In fact it's the very one you're using to see it's there in the first place. Catch 22.
Search for a little programme on-line called 'Unlocker'. It's free. Download and install it. Next time you get the 'unable to delete' message, right click the file and you'll see the option to 'unlock' it and then you will be able to delete it. Good luck.