Ok a few suggestions to get you started.
First is get antivirus up and running and do a full scan to eliminate virus and or trojan activity being the cause.
Next make sure they run a spyware checker like adaware to clear out the inevitable spyware on the system.
The last suggestion is to check out the various threads here about using a specified proxy as many have found this has greatly enhanced browsing speeds.
Hope one of these suggestions will bear some fruit.
Meantime post back once you have checked if any help improve things to let us know if they worked or not.
EDIT: GMTA th'engineer
