I am a professional developer with Microsoft, and have about 10 years of experience in the field of Computer Engineering. Most problems with PC's are the result of malicious software that infected a Computer. Examples of this are things such as malware, viruses, trojans, spyware, adware, root kits, and worms. In other cases, they are the result of stability issues due to crashes, freezes, or Hardware Issues. Some examples of hardware issues could be a dirty cooling fan causing the computer to overheat, or low memory due to lack of RAM, which could cause the PC to run slowly. You may think it could be a virus, or malware, or registry issues, but it could be something else entirely. As a computer layman, one may find it hard to really ascertain what exactly is causing the problems, without having a professional do an in depth analysis of your PC.
As professional computer repairs can be quite costly, I would recommend to you a Software that many of us in the tech department here at Microsoft use personally and professionally to diagnose and repair computer problems very quickly. It is a software called Reimage, from https://reimagefix.im .. In my personal experience, and in the experience of many of my associates, this is probably the best software ever created for those experiencing computer troubles. This is likely to remedy just about ANY problem your computer could possibly be having, and have it running like new. Reimage has won several awards, and been featured in publications such as PC Mag.
Personal Experience with https://reimagefix.im