I use freeware sandboxie.com... it's basically a virtual sandbox for your PC environment that keeps the junk out. I've used it for a couple of years and have never had a problem with spyware, adware, etc. It's clean, light and does the job. I clean my sandbox each day and my system stays fresh.
You can also use it to test out software downloads on your system without the software actually accessing your hard drive. If you don't like the software after you've installed it in the sandbox... simply clear the sandbox and your done. Nothing is left in your registry or hard drive.