Is the computer really slow? Could be a virus or something stealing all the memory.
If you install a second program on top of the first, it will only make it worst. They are basically fighting for memory and if they are scanning or using the same file at the same time, it can get corrupted. In my experience, running two programs at the same time is worst than the effects of a mean virus. Most programs use a real-time scan so they are always watching the computer. You get more than one going at a time and it's like a tug of war.
Best bet is always to run one scan at a time in Safe Mode. If you roll the system back to before the problem started, look at the log and see which files were changed/removed, it might indicate if a virus or adware was messing it up.