To add to what Usram wrote.
Try Ubuntu. It is very stable, easy to install and easy to use - it is NOT Windows however, don’t expect it to be.
You can download an .iso direct from the site, burn it and install it - note that Windows doesn’t come equipped to burn .iso files, you may have to resort to third party burners (Nero, possibly)
You can order a free CD which will take a few days to get to you. They mention up to 11 weeks but my latest one came in four days, which is pretty normal. I haven’t used it yet as I just upgraded the desktop a couple of weeks ago, online installation. Despite many warnings to the contrary it worked perfectly, just like it did for the last release six months ago.
There is also Wubi. You download it into a Windows folder - no need to repartition - and use it from there. I have absolutely no experience of this so couldn’t say if it would be suitable for you.
Also, as noted above, you can install WINE direct from the Ubuntu repository and run quite a few Windows programs in it. Again, I have no experience of it as I have never had the need or desire to run Windows programs on Linux. I have heard that games are a complete waste of time though.
As Usram mentioned, there is a “Live CD” option (actually, the ‘Alternative’ CD has this option on it too, I don’t understand why they bother with both.
You run this from your CD ROM. I’ve done this repeatedly on my Toshiba Vista laptop and it actually runs faster and smoother than Vista. I would try this for a bit to get the feel of Ubuntu, then install it alongside Vista if there is sufficient spare capacity on your hard drive - it has a very small footprint compared to Vista. That way you don’t lose your known computer and program while you familiarise yourself with Ubuntu.
The best thing about Ubuntu, IMO, is the support forums. The people there are much like the people here, friendly to a fault and keen as mustard to help out with any problems.
There is also a site semi-dedicated to help topics for the Ubuntu uninitiated that would be worth a look before you leap, Psychocats, that gives simple tutorials on how to partition and install, also some of the relatively routine bits and bobs that you need to do to get things the way you want them to be.
Over the past year the hardware support has improved immensely and installation become easier.