Hi, your comp shouldn't've caught a virus from an MS update, if you downloaded it from the official source: http://update.microsoft.com (Windows Update).
Have you, or anyone else who might've used your comp, opened any e-mails claiming to be from Microsoft? If so, that could be the source of the problem. Never open any such e-mails - delete them immediately. Microsoft never sends updates by e-mail.
Updates are obtained by the user (you) via update.microsoft.com so that their site can detect your system settings, to issue the relevant updates for your operating system.
To reduce the risk of viruses, be careful what you download, don't visit suspicious sites and download unknown files, and you should be OK. I don't use anti-virus software myself, because I don't take such risks, reducing the need for one. So that's one less program to have to load, which frees up system resources for better performance. Though I do use a firewall.
I know what you mean about formatting though, but you can take very simple measures to prevent having to do that so often. I haven't needed to format any of my systems for a couple of years now. But it hasn't always been like that - this is something of a personal record-breaker for me. Good luck!
Info & tools:-
http://www.microsoft.com/athome/security/protect/default.aspx -
Protect your PC
3 steps to help ensure your PC is protected.
http://housecall.trendmicro.com -
Trend Micro - Free online virus scan.
Information about firewalls -
http://en.wikipedia.org/wiki/Firewall_%28networking%29 (Full definition on the different types of firewall).
http://en.wikipedia.org/wiki/Personal_firewall (Personal [software] firewall).
http://www.zonelabs.com - ZoneAlarm - One of many good software firewalls that controls inbound and outbound connections from your computer, with lots of other useful settings.
http://www.lavasoft.com -
Ad-Aware is a program from Lavasoft that detects and removes software on a user's computer that is determined to be spyware. It also detects dialers, trojans, malware, data-mining, aggressive advertising, Parasites, Scumware, Browser hijackers, and tracking components.
http://www.safer-networking.org -
Spybot - Search & Destroy is a malware, spyware and adware removal program which works on Microsoft Windows 95 and later. Spybot-S&D scans the computer hard disk and/or RAM for malicious software.