Open up Task Manager, and then Performance Monitor. I am sure in there that opens up and shows disk activity. I don't currently run Vista myself, but do use it on other people's machines.
Another thing to do is again open up Task Manager, then look at the various processes that are running. Sort by the activity column, so those that are using the most of the CPU are at the top. That should give you a clue as to what is running and doing things.
Things to look for are:
Windows indexing
Other indexing...google, copernic, nero etc
Disk degragging (Disk Keeper)
AV scanning
Anti spyware scanning
Those are the most common for attacking your drive.
ALSO, DONT UNPLUG THE DRIVE IF ITS IN USE. If you do, you will most probably corrupt its partitions. I did this on a friends machine. I did not know he has Disk Keeper...it hard started to defrag my USB drive. I did not realize and just unplugged it...opps.....120Gb of data down the pan