"security issues and had to format it"?
That's BS.
I thought the same thing, too.
Let's say OS X has security issues and needs to get formatted, then where the heck does that leave Windows? LMAO.
I make $40 per hour dealing with Windows and that's what I would get with OS X repair/consulting. But in ten years, for OS X issues, I have made ZERO. That's how stable OS X is. I hope the OP doesn't go to Windows 7 thinking it will be more secure than any version of OS X.
The only security issues that I know of with OS X was with the first release of Leopard and how vulnerable it was in a Windows network. But that vulnerability could be shared with Windows Server. Apple pretty much got on the same page with Microsoft as far as talking to the LANs and WANs, and keeping hackers out. The krux of the security had to do with a hole Time Machine left in OS X. You want to make it easy to see back into time with Time Machine, but in order to do that, you also have to loosen the kernel, and that's not worth it IMHO. That's why I have Tiger. I will soon jump over Leopard and not take the chance and go over to Snow Leopard. However, Apple saw Leopard's issue with Time Machine and repaired it pretty well. For the average user, they simply don't care and all they really have to know is that OS X is and has been more stable than Windows.
Hackers hit Leopard again, more recently, but now it's not an issue as Apple has moved on and locked that vulnerability. MS went to the umpteenth fix on Server 2008 and we all know how safe that is.
I get to take a class next semester in finding vulnerabilities in OS X and Windows and I will get a better perspective by comparing the two. Last semester, we just kicked the SH** out of Windows and laughed at MS's official answers to those holes in their OS.
If I wanted to be even more secure, I would go with Ubuntu or similar, but next semester's class will work on trying to break that, too. My teacher is a certified ethical hacker so we will see, but with what little knowledge I have, there's no way to say 7 is as stable or safe as Snow Leopard.