Invalid Templates?

Sounds to me like a path setting or location is not set. When you create a new part or assembly or drawing SW is looking for the default templates for a part, assembly or drawing. I would check for the following ...
Open Solidworks and gotot Tools\Options and set your "File Locations" and "Default Templates" locations.
Check and correct these "File Locations": Templates Sheet Formats We have all of our Custom Templates and Sheet Formats in the same shared network location so everyone uses the same.
Also, check and correct the "Default Templates" location. This place has the three categories: Parts, Assemblies and Drawings. These Defaults are used when you are importing and maybe creating a new part, assembly or drawing.
By installing SW2008 it is most likely that these locations are pointing to the wrong places. For instance you may have installed into a specific location so the paths don't match up.
If you don't know what the correct location is Search your computer for files that end with .prtdot, .asmdot and or .drwdot.
Good luck,
~George

As George suggests, the file location paths are likely incorrect.
Did you install SolidWorks 2008 alongside the 2007 (or another) version and then remove the earlier installation(s)? If so, the file path for the templates (and other paths) which were migrated from the earlier version of SolidWorks would leave 2008 with pointers to certain invalid locations.
Per O. Hoel ______________________________________________

As George suggests, the file location paths are likely incorrect.
Did you install SolidWorks 2008 alongside the 2007 (or another) version and then remove the earlier installation(s)? If so, the file path for the templates (and other paths) which were migrated from the earlier version of SolidWorks would leave 2008 with pointers to certain invalid locations.
Per O. Hoel ______________________________________________ You should also make sure that the first time you start the program after installing it that you start it from the start menu not the desktop icon.
Bob