Here is the problem, i downloaded a .avi file which is around 700mb in size, it was downloaded through a p2p service (i know!) but i can't delete it, when i do try to delete it it says another program is using the file but it is not. My computer now also seems to be going very slowly.
Not only do you probably have some malware/scumware still lurking out there, but someprocess is keeping you from accessing that file. There are utilities that allow you to view and kill rogue processes.
Download and extract the free utility PrcView from http://www.teamcti.com/pview/prcview.htm
This utility is excellent for looking at what processes are running on your system. It will show you not only processes but threads that the Task Manager often hides from you, including spyware. It also can give you details on each task and kill them, if you so wish.
Installation doesn't modify the Windows Registry, so you simply extract it to wherever you want. This is the main reason that I still prefer it to CodeStuff Starter. I find it so useful that I put a shortcut to it in my Startup folder; I run it with Windows.
You can post here the list it generates, and we'll let you know what can safely be killed.