I tried using SiteAssist 3, and get this error message when it opens:
At line 451 of the file "C:\Michael Perry\....\WA_Preview.js"; tDim has no properties.
The error happens 2 times, once for each preview window in the extension. Perhaps this will help narrow down the problem. . .
ETA: Preview in CS5 works for a non-Administrator account on the same PC. Perhaps a permissions problem??
Edit 2: Further investigation shows that the above cited error occurs with the WA_Preview.js file that is part of the SiteAssist 3.2.3 extension -- the current version of WA_Preview.js does not throw this Dreamweaver error.