I believe you can do ASP.NET stuff with Mono
, though I'm not sure how SQL Server stuff would work (unless you're just running a server on a Windows machine and connecting to it with ASP.NET).
Windows programs won't run natively under Linux. You'll be able to use Microsoft Office files with the OpenOffice.org office suite (or some other office software, e.g. KOffice). You can also get Windows software to run under Linux with software like Wine