When setting up your computer you really should have 2 accounts.
- An Administrator account ( Windows 7 ,Vista & XP , Apple)
- A Limited User Account ( XP ) Standard User (Windows 7 , Vista , Apple)
Then for your day to day activities you should run in limited / standard user which will avoid you installing or running unwanted programs and applications that you have downloaded.
Virus / Malware require you to run in Administrator mode as they need to access system files , hence running in Limited will enable you to avoid activating these programs.
In Windows 7 & Vista the UAC is turned on and should be left on so that unauthorised changes to your computer are monitored.
This may be viewed by some as a nuisance by some but is a security measure you should follow.
If you have the 2 accounts you can also check if you experience software issues by swapping from one account to the other.