Unless you have downloaded the Beta/Release Candidate, or don't have Windows XP, you will have Internet Explorer 6. However, there is a new version available for everyone to download, for free, that upgrades IE6 to IE7.
Why do such a thing, I hear you ask! There are loads of reasons on this web page from Wikipedia.
But probably the most important is the security strength. IE7 Beta 1 was released at the beginning of the year, and if you have noticed, a lot of security fixes have been released for Windows XP Service Pack 2.
However, the ones related to Internet Explorer have been for version 6 only. According to Microsoft, many of the security flaws are already resolved if you have Internet Explorer 7. IE7 was already designed against these flaws due to its new operating engine, and so it is impervious to such attacks.
One example can be found on this web page from News.com.com
Therefore I recommend that you upgrade to the new version today!