When should I use a struct instead of a class?

MSDN has the answer:
Choosing Between Classes and Structures.

Basically, that page gives you a 4-item checklist and says to use a class unless your type meets all of the criteria.

Do not define a structure unless the
type has all of the following
characteristics:

• It logically represents a single value, similar to primitive types
  (integer, double, and so on).
• It has an instance size smaller than 16 bytes.
• It is immutable.
• It will not have to be boxed frequently.