I want to test my software on different Windows Operating Systems. I plan to do it using Virtual Machine software, either VMWare or Microsoft Virtual PC.
I would like to be able to test Windows 7, Windows Vista, Windows XP and Windows 98 in both 32 and 64 bit flavors, and possibly at differing service pack levels.
Where can I get the Images or full OS's, either free or by purchase, to run on either of these VM systems?
Update, June 2015: Microsoft is retiring the TechNet Subscription service and replacing it with the TechNet Evaluation Center. So romandas' answer is no longer valid. I'm moving the accepted answer over to Michael Burr, whose link still provides a good number of OS images, and for free as well.
You can get VPC images of installed OS's from Microsoft at no cost for a select number of OS/IE configurations:
There are a couple caveats - the images expire in a few months, then they put up new ones for download - so these really are for testing. Also their licensing might have restrictions in addition to the expiration date (I honestly don't know - I haven't used one of these in a long time).
Currently the available images are:
For other OS's, your best bet is something like the MSDN or Technet subscription mentioned elsewhere.