brand new imac going s l o w e r

Fed up with microsoft etc i took the plunge and got myself a new imac with an additional 4gb of ram in it. At first everything was great, but now my little acer aspire notebook loads Youtube quicker and it's a yawn waiting for web sites to load up. Something has obviously upset it!
Can any experienced mac users tell me the most obvious cause cos I'm not too sure what to try first?
p.s. a virus scan yielded no infections.

Mac have no virus'. Head to Applications>Utilities>Disk Utility.

You will use Disk Utility to repair permissions and verify the hard drive. Before your run this program, go to the Apple (top left corner) and pull down to Software Update to see if you have the latest and great from Steve, forget Bill now. After it has completed (may be many times if you have a fresh install) then Run Repair Permissions and let it run till complete which can take 15-20 minutes the first time. Restart and try life again. I have found Firefox to be a faster browser than Safari IMO.