Defragging is the most over rated thing i've ever seen lol.
The most likely cause for this slow down is a virus / spyware / adware.
Without a firewall, a pc can grind to a halt within a day of being connected to the internet, but even with a firewall, spyware can accumulate and gradually slow your pc down.
Visit http://www.computeractive.co.uk/downloads/
From here you can download: -
"Zone Alarm 7" (A free and very good firewall). Don't download this if you already have a firewall.
"AVG 7.5.446" (A free antivirus program). Again, don't download if you already have an upto date antivirus. If you choose to replace an old antivirus program with this one, download the AVG file, disconnect from the internet, uninstall ur old antivirus and install this new one. Connect to the net and download all available updates.
"Spybot Search and Destroy" (Anti spyware/adware). No matter what security you have on your computer, download this too. This will fix most of your problems, if not all of them (you will have more than one problem on your computer).
"Adaware" (Another anti spyware/adware program). Again, it's important to download this tool as it will remove what ever spybot can't (and vice versa).
Once downloaded, install and update each program (might take a while, but make sure that you do update them, this is important).
Perform your scans and check the files you want deleted (inn alomst all cases you will be wanting to delete all the dangerous files found, so right click and click 'select all' (or similar) and then 'fix problems' (or similar).
Good luck (I'll be checking this page to see if you have any further problems).
Dave
PR1N6L3M4N
March 2007