Definitely frustrating. All we can do is keep ruling more and more things out. Anything else unique you can tell us about your system?
Is it a dual boot configuration?
Is your root drive your C:\ drive?
Are you installing to the default directory or attempting to install elsewhere?
How much free space is available?
Other than SONAR X2 Trial and MC6T, have you attempted to install any other programs recently?
Are there any Microsoft services disabled by accident? Easy way to check this:
1) Go to the Run prompt by holding the Windows key and pressing "R"
2) Type in the following: msconfig
3) Click [OK]
4) In the System Configuration utility, click on the Services Tab
5) Look for services with "Microsoft Corporation" listed as the manufacturer
Any non-Microsoft services running in the background that maybe should be temporarily disabled? Easy way to check this is to do the above, but click the "Hide all Microsoft services" button.