Can't start any VMs using VMWare Fusion v3 on Mac OS X, nor upgrade to v4
Fusion uses lock files to prevent multiple processes from modifying the same files at the same time, as could happen if you tried to run to copies of Fusion (say version 3 and version 4) at once.  When Fusion quit abnormally, as it did when your laptop battery died, it left behind the lock files and you need to clean them up manually.

See this support article for how to clean up Fusion lock files.  If that doesn't solve it, do a Spotlight search including system files for files whose names end in .lck and if they look like they belong to Fusion, delete them.