Being a bit of a computer geek, I have a few tips to help prevent this:
-Anti-Virus
This one should go without saying. Make sure you have a decent anti-virus and make sure your virus definitions are up to date. Most Anti-Virus programs do this automatically, but set up automatic scans during nighttime, or whenever you don't use your computer, as it can slow it down.
-Delete your cache/history at least once every couple days
This is very simple, it helps to protect you from viruses accessing your personal information (credit card numbers/bank account numbers) if your anti-virus is compromised. It can also significantly increase your browsing speed.
In Internet Explorer->Delete Browsing History->(You should check all of the items to be deleted, but some are not entirely necessary, the biggest one is temporary internet files.)->Press delete.
-Router security
Make sure if you have a wireless router it has some sort of password protection. (WEP, WPA) I have seen cases of hackers being able to access your network (and your computer) and install a key-logger, which will save what you type for them to retrieve later. They can get your credit card numbers, your online banking info, etc.
-Phishing
When you go to any website, especially if you googled it or some other search engine, take half a second to glance at the URL in your browser to make sure it is correct. A common phishing technique is to copy a webpage and give it the same keywords and a near match to the URL as the original. Let's say it's your Bank Of America Online banking. The page looks just like the real thing, but the URL will be slightly different and when you enter your log-on info, it is sent to the malicious programmer, who then can access your banking info, etc., etc.
These are just a few of the things you can do to stop the most common of hackers, malicious code, and thieves.
If you see a charge on a credit/debit card that you don't remember making, contact your bank ASAP. Most banks can tell you exactly where it was charged, for what and when.
Hope this helps,
Chris