this is weird, it's been happening to me all day. My torrents will randomly stop and I get an error message saying something to the effect of "files in use by another program"
this only happened on downloading torrents not seeding torrents.
Windows 7
utorrent 1.8.1
I looked around in the task manager for what could be accessing the files but didn't see anything.
I also restarted my PC, restarted Utorrent, removed and re downloaded the torrent.
any other suggestions?

I had this a few times before in the past but IIRC it was down to some new AV I was trying and it kept locking the files out for scanning every now and again. It also happened to me repeatedly when I used to use Google desktop, it's indexing of the new files interrupted uTorrent.

Try using Unlocker on the files in question to see what is hooking into them to make them stop. You do need to use it immediately as soon as the torrent stops though or you will likely miss what's causing it. I'm not sure if it'll work on Win7 though. I know it doesn't work on 64bit for sure. Handy little app.

I told AV my program not to scan the folder where torrents go.
More to do with the slowdown it causes, rather than files in use, but I can see it would cause that problem too.