It is possible to replace that file but my suspicion is that it's not lsass that is corrupt. Here's Grif with scans for the usual malware but before you do this, be sure you have your files safe.

After your files are safe then you scan for pests and try the old XP REINSTALL method (google that!)

-> http://forums.cnet.com/7726-6132_102-5098912.html?tag=posts;msg5099421

Bob