A beta version of Windows 7 was leaked a couple of days ago, and while many people report the first beta build is very stable, it's definitely not a good idea to use this operating system as your primary OS.
In general, beta software is meant for beta testers only and ZDNet warns the leaked beta build doesn't have the latest security updates for Internet Explorer 8 yet. It's not possible to update the beta version of Internet Explorer 8 with the latest critical out-of-band security update, putting Windows 7 pirates at risk for attacks.
That’s the downside of running software from unauthorized channels. You see, the build zipping around the BitTorrent tubes right now is a random “daily build,” one of many that was produced during the run-up to the upcoming beta release of Windows 7. Anyone who would have had authorized access to that build (Microsoft employees, OEM partners, a tiny corps of trusted beta testers) already has authorized access to more recent builds (including, rumors say, the final beta release itself) that can be updated with a supported version of this crucial IE8 patch. So, logically enough, Microsoft’s security team doesn’t release an update for that leaked version, just as it didn’t release an update for build 6952 or build 6961. As a result, you’re vulnerable if you visit a compromised website using the unpatched version of IE8.