Hi I have just installed MCafee virus scan but since i did this my Pc is very very slow Why is this?

In all honesty McAfee is crap, and a resource hog.

Download AVG free, and Zonealarm free.

Disconnect from the net, and uninstall McAfee.

Install Zonealarm followed by AVG. Reconnect to the net, and update AVG.

http://free.grisoft.com/doc/1

http://www.zonelabs.com/store/content/catalog/products/sku_list_za.jsp?dc=12bms&ctry=US&lang=en&lid=dbtopnav_zass

Regards Howard :)