Microsoft are again urging PC users to apply the MS08-067 emergency patch issued last October due to an increase in attacks aimed at exploiting that vulnerability. In particular a new worm Worm:Win32/Conficker.A. has been noted as causing a rise in the number of attacks.
Once a PC is infected the Worm:Win32/Conficker.A. will patch the vulnerability to prevent the PC from being exploited by another worm or attacker and will also reset the system restore point to make it more difficult to recover the infected PC.
More details are available on the Microsoft Malware Protection Center Blog at http://blogs.technet.com/mmpc/archive/2008/11/25/more-ms08-067-exploits.aspx
If you have not yet applied this patch it is strongly recommended that you do.