If you are trying to run a shrink on the tempdb you must be in single user mode and ensure no other activity is occurring in the tempdb database. If there is any activity on the tempdb then the DBCC SHRINKFILE and DBCC SHRINKDATABASE Commands shall fail.

If you are doing this you must ensure that you follow the recommendations from Microsoft located at http://support.microsoft.com/kb/307487

There is also a support article for SQL Server 2008 which states the same as the above support article, but I don't have it to hand