Computer Woes

My computer is sick. Very sick. I think it has caught a virus of some sort and it really isn't up to its normal speed. We have antivirus software (McAfee) installed on our computer, but it didn't catch whatever it was, and it still refuses to acknowledge that there is a problem. McAfee's realtime scanning keeps switching off for some reason, and then freezes when I turn it back on. The worst part is how slow the computer is. I log on and come back five minutes later to actually open a program. Five minutes later that program might be open. Anything related to the operating system is as slow as molasses in Alaska in January. I haven't been online much, just checking email quickly and getting off, before random tabs start opening up when I least expect it. I'm getting really behind in things that need to be done on the computer because it is such a pain to be on. Most of the time I've been on the computer in the last week has been spent trying to rid the computer of the virus, but to no avail. I just don't know enough about the computer or viruses in general, and I don't know what websites I can really trust to help, and which will get me into worse trouble. One day last week I got a really big scare, when I turned it on and only got the "black screen of death", totally black except for a white curser arrow. I turned it off again and didn't turn it on again until much later in the day when one of the kids wanted to try it. It worked for him, and it has worked (in its slow, painfully limping way) since then. I'm thankful for small blessings. I'll be grateful to have the computer working properly again.