First regarding the Mouse, if u have a Wheel/scroll mouse then this could be because of a poor device. this can be checked by Pressing the scroll wheel hard for once when the screen jumps. It happened to me, changing the mouse did it all.
Now, coming to the basic question, this could be because of Bad Memory/ processor over heat or because of too many background applications or even a spyware/adware file causing the problem.
Assuming that you have Windows XP, the following should help you see some improvement in system performance and ofcourse identify the problem area [if not hardware].
First, click start - run, type MSCONFIG and press Enter. Click the Start tab, click Disable All and click Apply&OK. Restart the computer.
This should have increased the system resources.
If it is not resolved then try to perform a cleanboot by using info in the following link:
http://support.microsoft.com/default.aspx?kbid=310353
Also, download and run the Ad-Aware program to delete the spyware/adware files from system. These are the main reasons behind system problems. It can be downloaded from,
http://www.lavasoftusa.com/software/adaware
This should help.