my tempdb log is currently at:
Currently allocated space: 0,50 MB Available free space: -25,95 MB (-5190%)
How can this be minus? What should I do?
It might just be that a particular reading for available space is wrong (and the rest of the database is healthy). I have seen negative values like this after certain admin operations such as detaching a DB and reattaching it without its original log (i.e. using sp_attach_single_file_db) though in most cases this sort of thing would not be done against tempdb.
Also if the rest of your databases are fine (do check before proceeding, and verify you have good backups, in case the tempdb size misreporting is a symptom of a larger problem) then just try stop and restart the SQL services - I believe tempdb is recreated from scratch each time the services start.
See also the notes on this page about tempdb, specifically in relation to some tools misreporting the size (generally or after a shrink operation).