Start with Add/Remove programs. Get rid of anything you don't use, or don't think you'll use.
Next, go to Start > Accessories > System Tools >Disk Cleanup. That will bring up a window showing different catagories of files you can delete. Check them all and let it do it's job. It may take a while, if there's a lot of cached files and such.
Go to the same place above, and choose Disk Defragmenter, and let it defragment your hard drive. You should do this every couple months.
The external hard drive suggestion is a good one. Make copies of things you would not want to lose (like your pictures and such). You could move data that is not very critical to the external drive, and just fire it up when you need to access it. Otherwise leave it off.
Buying a new computer easier? That's like saying 'my car is dirty, would it be easier to buy a new one?'. A computer is a lot like a car. There are maintenance practices that need to be done, and unfortunately, they are not widely known by average users. Most users think they can just turn the computer on and surf away with no problems. Nowadays, you need a firewall, antivirus program, spyware removers... running a computer is an active involvement, and if you're not active in keeping it healthy, it can get ugly, and be VERY difficult to fix.