Where can I get images or full Windows OS's to run in Virtual Machines [closed]

Question

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?

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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 So romandas' answer is no longer valid.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.

Answer 1

Edit: Apparently, my original answer of using Technet is incorrect if you plan on testing custom code with the Technet-provided products. I'm not going to comment on whether it's illegal or not, since I Am Not A Lawyer, Nor Do I Play One On TV, but it is clearly a violation of MS's license agreement to do it. So, if you want to test custom code on MS products, apparently you will need to grab one of the MSDN subscriptions. My apologies for any confusion this may have caused.

Less expensive than MSDN (over $1000 I believe) is MS Technet Plus ($349 for download-only; $249 to renew). Since you are only asking for operating systems, not developer tools. Technet includes everything MS has except Visual Studio and related. I use this for building systems within my VMWare testbed. 10 licenses per OS.

There are 32-bit and 64-bit versions of all their OS and servers, plus legacy stuff going back to MS-DOS.

Information here: http://technet.microsoft.com/en-us/subscriptions/default.aspx