Administrator accounts have more rights than any other account on a Windows system. Also, they alone have the power to give themselves additional privileges.
By default, admins can edit the registry, create and manage user accounts, assign disk quotas and permissions, take possession of files and folders, install drivers, join domains, etc etc.
Because administrator accounts have so much power, it is ill-advised to perform your day-to-day activities running as an admin. The increased user-rights are not necessary, and should you catch a virus, it may be able to spread into your system more aggressively because it was launched by an admin.
Inexperienced users should not run as admins. The likelihood of you accidentally nuking your system is much higher than if you ran as a normal user.
