Can't re-try demo software

A few years ago I must've demo'd a bunch of diet software on my laptop. I really want to buy something now and of course all the demos have expired, but I can't reinstall them as something must be coded into my system saying they've already been on there. All I get is a screen saying my demo has expired and it give me a buy button. I can't figure out what to delete or edit on my machine in order to be able to pick one of them out to buy.

I know this is designed so that people won't keep extending trial software, but come on - I did this in early 2010!

Any ideas?

Thanks!