Desktop computer help

My families desktop computer is acting up. Basically it is so slow, it is unusable in normal mode. This comes after the motherboard crapped out a few months ago and we brought it to one of our computer savvy friends to look at it. He said he replaced the motherboard and added RAM. The computer worked great for about two weeks, when it started doing this. I am now running it in safe mode and trying to back up the hard drive on an external drive, but I think the software won't install while its in safe mode.
Any help or advice?
Any help or advice?