What causes erratic scrolling?

I have the same problem too on Vista Home Premium. This problem happens particularly when I'm e-mailing within Hotmail or sending text within a box ie on Facebook. The cursor will move up a line on its own accord or move to the beginning of the paragraph. At the same time the box Im typing within minimises itself and I have to keep reopening the box to continue typing. I'm wondering whether this could be due to a virus although I keep my computer updated regularly and it's scheduled for a daily scan. Can anyone shed any light or know of any fixes?