Sandboxie enables you to easily sandbox your browser and other programs, it runs your applications in an isolated abstraction area called a sandbox. Under the supervision of Sandboxie, an application operates normally and at full speed, but can’t effect permanent changes to your computer. Instead, the changes are effected only in the sandbox.

For those too lazy to set up a full on vm image for testing stuff, this is a pretty good alternative.

Registration is optional but there is a nag screen after 30 days.