Yes, Kaspersky on both machines.
Kaspersky is known to cause problems with Adobe Creative Suite and other Adobe installs, particularly downloadable digital editions. Also, there were some well documented problems with older versions of Kaspersky on Windows 7, especially 64-bit versions. Kaspersky 2009 is not compatible with Win7 at all, and 2010 is compatible, but there were lots of problems early on. If you don't have the latest versions of the AV software, it would be best to make sure to update it (not just the definitions, but the application itself), or move to Kaspersky 2011.
I'd actually recommend a different product, but to each their own...
It is also possible that something is stuck in the Windows Installer. Major Geeks still hosts the old Microsoft Windows Installer Cleanup Utility here:
http://majorgeeks.com/Windows_Installer_CleanUp_Utility_d4459.htmlIt can sometimes help fix broken installs, though I don't know if MC uses the Windows Installer functionality so it isn't clear if this is relevant to these problems. Also, Microsoft pulled support for that utility because it can cause damage if used incorrectly, and it is known to cause problems with Office 2007 in some cases.