I didn’t know a letter can be trademarked. If let’s say Xerox decides to get into the video game business, can Microsoft sue them for trademark infringement?
Xerox is not "X". You may be interested in doing some research in this area what is trade markable and also the procedure. Even if the trademark is registered it can still be challenged. Other interesting things that can be trademarked include colours and sounds, even scents
In the EU there even is a "3D trademark". One famous example is the LEGO mini figure, which LEGO uses to sue anyone that tries to sell any remotely similar LEGO compatible mini figure.