Something is going on in the background.  Start with a full scan of the
drive with an up to date anti virus program followed by a spyware hunt:

Spyware/scumware/adware are causing numerous system problems.  I recommend
you download and use the following programs and consider removing everything
these programs identify.  Be aware that there are frequent updates to these
programs and you need to check for them before every use.

AdAware:  http://www.lavasoftusa.com/software/adaware/

CWShredder:  http://www.spywareinfo.com/~merijn/cwschronicles.html (download
link near the bottom)

Spybot:  http://www.safer-networking.org/

If the problem continues, post back to this thread