Anytime you have some odd computer issue, you should never ignore malware as a possible cause. All the same, anytime I have to deal with a computer that has performance issues, I usually start by running hard drive and memory diagnostics. If the system passes, I run TDSSKiller, ComboFix, and MalwareBytes AntiMalware in that order. It's also a good idea to follow up with an online scan such as Eset's online scanner.