I had someone mention they weren't able to run imvu in two Sandboxie windows because the imvu installation was corrupted. I suggested she try downloading the actual exe file, rather than the client download manager file, but as youtube doesn't allow us to post links there, I offered to post the link here in my blog.
If you ever need a specific imvu version, this link can be edited to download whichever version you're interested in:
http://static-akm.imvu.com/imvufiles/installers/InstallIMVU_498.0.exe
Simply change the version "487.0" in the above link to the version you are looking for. Be aware imvu disables versions that are a certain age or older.
