You sound like you may already know this but you can try doing a telnet to the sql server machine on port 1433:
telnet <hostname> 1433
If you get a blank screen (open connection) then your service is running on that port and there's no firewall in the way. If that's not working try:
telnet <hostname> 50235
If that works, then as the previous answer suggests you're configured for dynamic ports instead of 1433 (dynamic is the default for Express edition). You can change that in:
SQL Server Configuration Manager > SQL Server Network Configuration > TCP/IP (Right Click > Properties > IP Addresses.
In the IPAll section set TCP Dynamic Ports to blank (not 0) and enter 1433 for the TCP port)
Hope that helps.