Listen to Paedric, if you are absolutely certain, hell if I have even the slightest doubt I reformat, but for some that's a little extreme.
I recommend installing
Webroot SecureAnywhere it works from what is called "the cloud" so it keeps you automatically up to date on the latest threats without bogging your system resources down. It's all in one protection.
As for your question, while changing your IP address may divert some attention it does nothing in the grand scheme of the Internet where people and scripts are looking to exploit any computer they can.
There is software that will mask your true connection, but who or what are you attempting to hide from? If you're looking to hide from an attacker on a particular game masking your IP may or may not impact your ability to play considering this involves proxy servers and firewalls.
What I would do (Taking your word as what actually happened)
1. Completely reformat your harddrive, and if you're really paranoid flash your bios lol
2. Get a router and lock it down with password protection and if it's wireless be sure to set up encryption as well
3. Use Webroot on your PC or some other basic antivirus, anti-malware app that isn't too heavy on your system
4. Don't get mouthy with people who can crack you in the first place
A little tip regarding your IP, most Ethernet cards support an entered value for your NIC's MAC id. Simply change your MAC id and depending on your ISP you're computer will look like a new device and request a new IP.
But another thing to note about changing IPs, when you release your IP that IP goes back into the pool available for someone else. Think about that, what goes around comes around.