Before you format the drive,
Are you using Norton if so dump it, but make sure that it is totally uninstalled by going into search and typing in Norton (Delete any remaining files showing Norton that come up)
Unplug the internet connectoion first
Then run Disc Cleanup then Defragmenter.
Go to Programs ,Accessories, System tools etc
Restore the internet
Go to www.filehippo.com
and download Avast Anti Virus and Spy Bot , install updates and run them both.
If your machine is not running a lot quicker then go to www.belarc.com
and download their Advisor program run that and it will give you a profile of your pc telling you what Ram is installed
For a reasonable performance under XP you need 2GB Vista take around 4GB
For a online check co to www.crucial.com
and run their memory checker , it will tell you what memory is fitted and what upgrades are available