A computer which accesses the internet needs three security components:
1) Antivirus
2) Anti-spyware
3) A firewall
http://www.theyeshivaworld.com/coffeeroom/topic/vista-windows-explorer-freezing-up
To see a comparison of free antivurus apps, I recommend a look at this page:
http://download.cnet.com/2797-2023_4-371.html?tag=mncol;txt
A computer that suddenly shuts down can be caused by many issues, including hardware.