If you are a little paranoid, then I suggest you check out the link to DropMyRights (in my signature below) - Browsing the Web and Reading E-mail Safely as an Administrator.
Whilst browsing or collecting email, etc. if you get infected then the malware by default inherits the same permissions that you have for your user account. So if the user account has administrator rights, the malware has administrator rights and can reap havoc. With limited rights the malware can't put files in the system folders, create registry entries, etc. This greatly reduces the potential harm that can be done by an undetected or first day virus, etc.
Don't let malware inherit administrator rights by default and get established in your system folders and registry.