How do I secure separate login accounts from viruses?

[keyven]

Essentially, can two login accounts on one PC - lets say "Andrew" and "Ben" - be secure and isolated from each other?

Let's say "Andrew" has sensitive personal information and should not have direct access to the Internet.

"Ben" can connect to the Internet but cannot share data directly with "Andrew".

Can I accomplish this with minimal difficulties?

[Geek-9pm]

Yes. Assume you are using Windows something lather than Windows XP.
Any user who has administrator level can  get into the other user accounts.
So in your example you shall lower the level of authority of one or more users.

Here is a general reference .
User Account Control Overview - Vista
Or:
User Account Control   Windows 7
Does that fit your needs?