Source : .NET Runtime
Description : .net Runtime version 2.0.50727.3603 – Fatal Execution Error (7a09795e) (80131506)

Cause:
Symtoms:

1. As above. seen on .net version 2.0.50727.3603 Note: you version number may be different.
2. Backup Exec Server Service may be stuck / hung in the "starting" state.
3. .net 2 cannot be removed or service packed.

Fix:
1. Download this, then apply the relevant 32 or 64 bit hotfix.
2. Reboot the affected machine.

Comments:

Credit to Andy Dorrian
MS KB 963676