First, go to Run>Programs>Accessories>System Tools, then click on Disk Cleanup. Tell it to empty Temp, Temporary Internet Files, Recycle Bin. Then, if you have old programs you don't use any more, go to Control Panel>Add/Remove and get rid of them (uninstall). After all that, go back to System Tools and run Defrag (Defragment Drive....choose c: drive). I suspect you haven't done this for awhile, and it may take some time for defrag to finish. When it is done, reboot and try your browser again. It should be much faster.
*** EDIT - When done, follow Merlin's suggestion and get spy sweeper (or AdAware, Spybot, or something), and run it to get rid of "nasties" (spyware, malware, etc) END EDIT ***