How many anti-malware programs do you have running?
One or two should be enough - have a look at this guide:
PC Safety and Security--What Do I Need?
Remove all startup programs - except your anti-malware and firewall.
To do that:
Click Start > Programs > Startup. Drag the "unwanted" programs out of that folder and drop them another place in the Programs menu.
Disable "System restore" for the ext. drive.