For all 3 of my machines, Autocad LT 2010's default start in directory was a usercache folder. I took off the usercache part leaving only "C:\Program Files\AutoCAD LT 2010\" and suddenly it works fine. This is done by right clicking the ACAD shortcut. As a test try launching acadlt.exe directly from the folder.
This this change fixed all 3 of my Parallels 5 ACAD installs.
Thanks!! This dit it for me! No need to reinstall anyhing. It works for full AutoCAD 2010
Specs:
MacPro 1.1 10GB RAM
Parallels 5
Windows 7 Utimate 64
AutoCAD 2010 64