How can I change the behavior of my mouse in Windows XP?

About two weeks ago my mouse started to work differently on one of my PCs, although I didn’t change any of the settings. When I click on a word, e.g. in a browser text box or when using Wordpad, it automatically marks a whole word or even a whole paragraph.

I went to Settings – System – Mouse and found nothing that helps.

Any ideas?

What could have triggered this?