I came across a nice little tool while searching the other day. This tool, called YAPS (Yet Another Port Scanner) is portable and works well (tested on Windows 7 x64 and Server 2008 R2).
You can grab YAPS from SteelBytes here: http://www.steelbytes.com/?mid=19
A little information from the site about YAPS:
Fast TCP/IP port scanner.
- Supports simultaneous connecting to many targets
- Supports command line and GUI mode
- Customisable timeout
- Can scan range of addresses, or single address
- Can resolve addresses
- Includes names for well known ports