Building a computer is not difficult. There's plenty of illustrated 101's in the Internet. The most toilsome part of it is to hide the cables neatly.
Installing Windows is not difficult, either. Again, lots of tutorials available in the Internet.
The two above don't require extra skills or professional tools. All you need is some time, working space and care especially with static electricity.
Fine tuning Windows for e.g. gaming can be difficult. There are some good sites, like
blackviper.com with tutorials and even ready made setups.
Maintaining it that way is the most demanding task which calls for some knowledge about what you're doing. Unfortunately, that's the part you can't buy or download. There are programs that can help, but you'll have to figure out which ones to trust and how to use. Most if not all of the 'One-Click-Maintenance' are like an Old West Medicine Show, having more bling and hocus-pocus than real effect and possibly even adding more background processes than it disables.
