Extreme Slow SQL Server 2005 And High CPU Usage - Casting Of Values

Hello sql and .net gurus :-)
I have a problem with my website www.eventguide.it. It's completly developed under .NET 2 and SQL Server 2005 Express. My problem is the folowing:
The server is a Intel 3Ghz HT processor with 1GB Ram. No other page on the running system is a CPU consuming site. We optimized the SQL statements, the code, the caching and many other parts of the website (pooling on SQL access), but the SQL Server uses about 50% to100% of the CPU and about 400MB RAM all the time. The whole site seems to be very, very slow. In fact there are many of SQL operations on every page request, but we cache a lot of them in different ways (page output caching, application caching). So I don't understand we have so much performance problems. Any suggestions for optimised code in general? I read nearly all of the MS .NET performance papers - but real world experience is the missing part :-)

It is better to cast the values of a SQL reader like this
Dim String1 as String = Ctype(DataReader.item(0), String)
Dim Integer1 as Intger = Ctype(DataReader.item(1), Integer)
or like this
Dim String1 as String = DataReader.item(0)

Dim Integer1 as Intger = DataReader.item(1)

Thanks a lot for your help!
FOX