Ask the prospect what database his applications require. If its MS SQL Server, then you are out of luck.
Its highly unlikely they will be able to port the MS SQL Server based application over unless it is *very* simple. MySQL and SQL Server are different in many ways ranging from simple things like the use of LIMIT to lack of sub selects, triggers, stored proc differences, etc in MySQL.
If they wanted to port, Postgres would be an easier target.
Chances are they simply want to run something with as little change as possible, which in the case of ASP + MSSQL combo, as you said, means a W2K host.