While the previous comments are certainly true that there is no panacea, there are free tools that can be helpful. You can download free memory diagnostics from the Microsoft web site. You can download free disk diagnostics from Western Digital and Seagate. I use Seatools from Seagate. And there are tools for diagnosing Windows problems too. In addition to your antivirus software, Malwarebytes is excellent for finding malware often called "pups" (Potentially Undesirable Programs), and SFC is a very good checker for corrupted Windows files. If you have something that doesn't work despite your best efforts (e.g. sound, Internet, CD/DVD etc.) you can use one of the free versions of Linux - I like the Mint and Ubuntu versions that you can put on a disc or flash drive. If Windows is irreparably damaged, you can use these Linux discs to recover your data too. These tools don't cover everything, but they do address many of the more common problems people have with computers.
Sorry if this seems a bit complex, but these are the do it yourself tools you can avail yourself of if you want to save yourself $100 or more at a repair shop, and have the time and will to pursue it.
Good luck.