It's the mods keeping an eye on you, 'cos they've got you down as a trouble maker. Some cunt told them you've got a pussy shot as your avatar. There'll be a black helicopter along in a minute, you mark my words.
I'm not sure that you quite grasp this; let me explain...
What's happening is that your computer is trying to be helpful by pointing out your spelling and punctuation errors. When your words get underlined, that's the programme indicating a mistake. Much like your teacher used to do in school. It's just saying, "Here's a mistake" and "Here's another mistake" and "by golly here's another one" and "Now there's more mistakes here" and "One more here" and "Yet another mistake...", or "You've done this wrong", and "Oh my goodness, what a cock up this is!". But instead of the computer being inconsiderate and causing annoyance with phrases like; "You've gone and done it again, get a grip!" it just puts this little red line under your words. Where some words aren't underlined with the little red lines the computer is suggesting that they might be correct. Although it doesn't guarantee that they actually make sense everytime.
I suspect that your PC has downloaded some updates from Microsoft, which included this little extra feature. Might be worth checking out the Microsoft website to see if there's a help guide showing how to turn it off.