If the computer's original condition was at the SP3 level, it would be
impossible to uninstall SP3.
To OP: If you absolutely need to run SP2, your only course of action is
a Clean Install, using an installation CD at the SP2 level (or below and
upgrade to SP2).
The larger question is why do you want to use SP2? Support for SP2 (that
is, critical security updates) will end in 2010. However, SP3 will be
supported through 2014. What specific issues do you have? 99.999% of
software that is designed to run with SP2 will also run with SP3.