Hi mepb.
I had a really bad experience with Returnil 2010, but since that time it has probably evolved into something less risky, but either way I would start off by trying the 2008 version, which is tried and tested, and has never given me a problem of any kind.
If you do try it, I would suggest watching the installation very carefully, and "skip" the "Virtual Partition" (there may be a box to check/uncheck, can't remember), and opt for the "Memory Cache" method as opposed to "Disk Cache".
This will give you Returnil in it's simplest form, which you can have start with Windows or not. Not, might be best to start with.
All you need to remember is that anything you do on your System Drive while Returnil is running, such as installing software, updating A/V, installing Windows Patches, making new bookmarks in your browser etc., will all be gone when you reboot.
You can of course save downloads to another partition, hard drive, or thumb drive.
For that reason, I use Sandboxie for browsing, with it's configurability (is that a word?) to save passwords, bookmarks etc while it's running, as well as being able to update Avast. I'm sure it automatically makes a clean install path for other AntiVirus Programs as well.
There has been a lot of discussion on here at various times on this type of software, and I've carried out a search for relevant threads.
Returnil Threads:
http://www.google.com/cse?cref=http%3A%2F%...um.piriform.com
Sandboxie Threads:
http://www.google.com/custom?hl=en&cli...m%2Fsscse10.xml
Hope that helps, and there's a link for Returnil 2008 in my signature.