There's a scumware plague at the moment. All it takes is a visit to a pushy web site or a "loaded" shareware install and next minute your Internet Explorer homepage has been changed, your default search setting altered, unwanted ads pop up on your screen and worse.

If you use Windows 2000 or later my top recommendation for safe browsing is a free program called Sandboxie [1] that creates a special contained "sandbox" environment on your PC. While browsing within the virtual sandbox provided by Sandboxie you are totally corralled off from other parts of your PC. So any files you download are isolated to the sandbox. Similarly, any programs that are executed only do so within the sandbox and have no access to your normal files, the Windows operating system or any other part of your PC.

Usage is remarkably simple. To start a sandboxed browsing session you just click the Sandboxie icon from the Quick Launch tray and this will launch your default browser in the sandbox. You can then use it in the normal way to browse to sites or download files.

If you downloaded a file it will install normally but again will be corralled off from your real PC. Anything it writes to your hard drive, any changes to the Windows Registry or changes to the Windows startup will be held in a separate area within the sandbox. Similarly, any new processes running in your computer memory will be sandboxed.

After you have finished browsing you can right click the Sandboxie icon and delete all sandboxed files and processes and your PC will be returned to the same state it was in before the browsing session. If you want retain particular downloaded files you can save them permanently before clearing the contents of the Sandbox.

The advantage is clear: any spyware, trojans, keyloggers or other malware products that infected your PC while browsing will be eliminated.

Sandboxie works fine with all browsers but requires Windows 2000 and later. It can cause problems on some PCs so backup before installing.

Source: http://www.techsupportalert.com/best_46_free_utilities.htm#1

See, your AV will still be monitoring whatever is being downloaded - whether in a Sandboxed mode or NOT. Sandboxie won't be able to tell on it's own whether you downloaded a virus/spyware - but what it WILL do is even if you somehow manage to run the virulent code, once you close your browser, all the infected programs will get restored to their original non-infected state. That much is guaranteed by Sandboxie...