There is a new Internet Explorer with tabbed browsing, magnify and increased security features. This is a Beta version so you are experimenting to some extent.
Click here for more information: http://www.microsoft.com/windows/IE/ie7/ie7betaredirect.mspx
In terms of rolling back if you don't like the new version...
If you have XP running it will normally set up a restore point before installing software like this. I just checked and it did that automatically on my install. To restore to the old IE-6 you would just goto Programs, Accessories, System Tools, System Restore and restore to a date prior to the date you installed IE-7