Go through all the normal things on this one I am afraid :-(
Run spyware/adware scans
(Search for Adaware/Spybot on Google.co.uk)
Check enough memory is free in Task Manager
(Alt Cntrl Dlt - Performance)
Once you have done those then you'll want to check the connection etc filters and line problems. May be a case PC needs a good tidy up. A bit vague I know but should help.
There can be many causes of this...