You need to determine if the computer itself is slow or if it slows down while accessing the Interned.
If it slows down while accessing the internet. do you have a router involved and can you bypass the Router for test purposes? If you have a wireless router is it encrypted to keep the neighbor from getting on to your router and hogging your bandwidth.
Are thee invironmental factors where the compute slows down while the Microwave oven is on or does it slow down while some one is using a cordless phone nearby?
Did you recently download any software or updates?