Back up important data. First make a catalog of all your important pieces of information e.g. Documents, Videos, Images, Applications. Any important bits of data that you do not want to lose, then copy them a CD/DVD or external place.
Download spybot search and destroy- this application is for searching for spyware on your computer and will allow you to see which applications will startup when your pc starts up.
Start spybot, select menu-> Mode-> advanced;
On the left column select tools.
Select system startup. This screen gives you a list of all the applications which start up when your machine which starts up, spyware inserts itself into this list so that it can startup when ever your computer starts up.
Find out which one is the source of the infection and untick the check box. The simplest way to do this is it google the executable name which can be found in the command line column.
Restart your machine.
Check that the application is no longer started up