I have tried TFS for few months using the trial version and got stuck when it told me that the trial period was over because TFS does not notify you beforehand that your trial version is about to end (as any other software usually does).
I did a quick search on google and found a TFS trial extender that worked fine for me. It extends TFS for an extra month which should be enough to buy your licence. Here’s the link:
Note that according the guy who did this utility, you can extend it only once.