I have been using ILDASM a lot for past few days when I encountered this bug. This problem is with ILDASM shipped with .NET 1.1 SDK.
If you open an assembly built with .NET 2.0 and try to disassemble it, it throws error “error : Failed to open meta data” which is valid because we trying to disassemble an assembly built on 2.0 with ILDASM for 1.1. But on clicking OK button, without closing ILDASM, try to delete or rename the assembly you are trying to disassemble. You cannot do it as IDLASM doesn’t release the lock over the file its trying to disassemble which it failed to do so.
Workaround for this is simple, just close ILDASM or open another assembly in ILDASM and it’ll release the lock over this file. Then you are free to delete or rename your assembly.
Bug for the same has been logged to MSDN Product Feedback.