According to this article by washingtonpost.com hackers can temporarily destabilize Windows 7 PCs due to a security flaw.
Here is the link to this article
http://voices.washingtonpost.com/securityfix/2009/11/microsoft_confirms_windows_7_s.html

----quote----

Microsoft warns of Windows 7 security hole

Microsoft has confirmed reports of a security flaw in its Windows operating system that hackers could use to temporarily destabilize Windows 7 PCs. The software giant also acknowledged that blueprints for exploiting the flaw are now available online.
----quote----
Information quoted from http://voices.washingtonpost.com/securityfix/2009/11/microsoft_confirms_windows_7_s.html

Here is a list of flaws according to this www.aspfree.com article
Here is the link to the article
http://www.aspfree.com/c/a/Windows-Security/Windows-7-Known-Security-Flaws/2/

1. Problems with Virus Scan Software
2.Hidden File Extensions
3. Reduced User Warnings

Here are more flaws
Here is the link to the article
http://arstechnica.com/microsoft/news/2009/02/the-curious-tale-of-windows-7s-uac.ars

1.Windows 7 UAC flaws

----quote----
The basic purpose of UAC is to annoy you when your software needs Admin privileges. The reason for this is simple: a lot of Windows software demands Admin privileges not because it needs to be privileged for everything it does, but rather because it was the quickest, easiest way for the developer to do some minor task.
----quote----
Information quoted from http://arstechnica.com/microsoft/news/2009/02/the-curious-tale-of-windows-7s-uac.ars

According to this link "Windows 7 auto-elevation mistake lets malware elevate freely, easily".
http://www.withinwindows.com/2009/02/04/windows-7-auto-elevation-mistake-lets-malware-elevate-freely-easily/

It was reported by eweek.com, that Microsoft has issued " 13 security bulletins, and span Microsoft Windows, Internet Explorer, Office, Silverlight, Forefront, Developer Tools and SQL Server. Eight of the bulletins were given a critical rating, Microsoft's highest severity classification."

For complete details of the 13 Security Bulletins, kindly visit the link below:

http://www.microsoft.com/technet/security/bulletin/ms09-oct.mspx