I was reading reviews for my firewall (PC Tools) and most say that it's, er, not enough, so I decided to get an installer for another one - Comodo. I haven't installed it since I'm not sure if using two firewalls at once is okay. Is it? Or do I have to uninstall PC Tools?

You should never have 2, or more firewalls, running at the same time.
They will conflict with one another, possibly rendering both inoperable.

The same applies to, Antivirus programs.

okay. thanks.

Using two software firewalls on a single computer could cause issues with connectivity to the Internet or other unexpected behavior. Further, running multiple software firewalls can cause conflicts that are hard to identify and troubleshoot. Only one of the firewalls can receive the packets over the network and process them. Sometimes you may even have a conflict that causes neither firewall to protect your connection. However, you can use a hardware firewall (your router) and a software firewall (Kerio or ZoneAlarm) in conjunction. For more information see "The Differences and Features of Hardware & Software Firewalls".