I'm thinking this has to be on topic, because I only use it for Internet and Photoshop.
I am forced onto my laptop again because my desktop is dead. It is a Dell with a two core T7600 @2.33Ghz with 4Gb RAM and is about four or five yers old. It is running Win 7 Home Premium.
I have CS5 installed on it, and used to be able to use it reasonably comfortably. Nowadays I can barely even open Photoshop and Bridge, and it is sloooooow! ISTR someone talking about a recent Win7 update that slowed some systems down. Does that sound right, or do I need to do an OS clean install? The latter would be no hardship as I don't have too many applications installed on it.
What display interface (GPU) does that system have in it? It is possible Windows Update delivered a bum driver, though that's not that likely as usually with laptops one has to get drivers from the site of the manufacturer of the machine (Dell.com in this case).
Forget about defrag for now. That's boilerplate stuff it seems everyone says, and it never fixes anything. Windows 7 normally keeps its disks defragged on its own, and disk fragmentation even if extreme is only going to impart a minor slowdown - it sounds like you have something night and day major going on.
And don't do a "registry cleanup", whatever that means. Running some piece of software someone somewhere else dreamed up is akin to pouring water through the machine to clear out debris.
One thing to do, especially if you haven't used the system in a while, is to check the Windows event logs for events indicating possibly the hardware is ailing or failing - for example recoverable disk errors.
What's running on the machine? How many processes are showing in Task Manager?
Have you installed software from Google or Yahoo (e.g., toolbars)? Those are generally awful - akin to malware.
How technical do you want to get? There's a good free tool called AutoRuns for checking just what software is starting on your system in the various different ways. http://technet.microsoft.com/en-us/sysinternals/bb963902
Trevor, make sure you don't put it off too long... Or next time you need it you'll be in a bind again.
You might want to check out my book on the subject of setting up and tuning up Windows 7 some time.