computer wont let me past welcoming screen

When I start my computer it takes me to my login page where I succesfully can log onto both of my accounts, the as the welcoming screen comes up a pop up open reading Slui.exe - system error Program cannot start because slc.dll is missing. I hit ok and another pop up opens saying explorer.exe - system error and the same think program cannot star because slc.dll is missing. i click ok on this one and I log into a black screen showing just my mouse the only thing that works is control alt delete help!

Like 4 years roughly,vits been running smooth and I got on earlier and it just crashed and this is happening?

Hmm.. I am using another computer because my other will not let me on so how am I supposed to download it onto the problem computer?

sorry guess I wasn't clear enough, my fault. what I meant was when it takes me to my accounts page I log in put in my passwords and they like..register I guess, it lets me login basically. brings me to the loading welcoming screen and then the popup comes up and the next screen that comes up is a black screen, instead of my desktop screen. All i have is my mouse and the only thing that I can access is ctrl+alt+dlt

ya I have a boot disk, can't get into safe mode during previous attempts, but I'll check tomorrow I'm going out so I'll check that later and message you back tomorrow