OK. Lots to cover here.
Firstly, firewalls have nothing to do with encryption. Neither does mIRC, IRC, or computer security in general. A firewall is a program or a physical peice of hardware that monitors your incoming and outgoing connections and denies or allows those connections based on various rules. They're important as an added layer of defence for improved computer security, but in general they'll do little to stop malware, especially if it's being run as scripts on a peice of software which you've specifically allowed to make connections (ie. mIRC).
I'm not entirely sure I'm following you but it sounds like you bought a "firewall" off someone after they broke into your computer? Using software given to you by someone who intruded into your computer is like buying a burglar alarm from a burglar. You should never use software suggested by anyone you meet on the Internet unless you're absolutely sure it's legit.
First things first: Check out the
Anti-Virus and Firewall Resources thread for links to legitimate firewall and anti-virus software. You'll want to run at least a couple of the AV-scanners listed in that thread (the free ones work just fine) and also at least one of the Trojan Removers. You might also want to look into deleting all your mIRC scripts and only downloading them in future from sites which check submitted scripts (for a brief list take a look at the
Scripts and scripting information for mIRC section of this site's link page).
Steps to prevent future infections and hacks:
1) Run virus-scans every week or two using any of the programs listed in the above link. The free ones should work fine.
2) Always run firewall software. Use one of the ones linked above. The free ones should work fine.
3) Regularly check you're using the most up-to-date version of any software you use. Particularly your web browser, email client, IM clients (MSN etc.), IRC client (ie. mIRC), AV software, firewall software, and of course Windows itself.
4) If you're using a newer version of Windows (2000, XP, or Vista), consider setting up and using a Limited User Account for your everyday computing activities. You only need to use a full Administrator user account when you're doing things like installing, updating or removing software. The rest of the time a Limited User Account will usually do and in the event that you do get hacked or infected it will help prevent anyone from doing especially damaging things to your computer.
5) Consider also getting a hardware firewall and router if you don't already have one. It's another layer of defence although of course this is something you can't get for free and it can be complex to set up if you don't know what you're doing.