userport.sys
Windows NT 3.x/4.x/Win2000/XP driver for direct I/O port access

userport.sys is a driver which enables direct hardware I/O port
access. Because TwinAVR uses direct hardware access for speed
reasons, the program doesn't work under NT systems without a
special driver such as userport.sys.

Note that under Windows 3.x (yes, TwinAVR can run under Windows 3.1),
Windows 9x and ME you don't need a special driver. Under these
systems the direct hardware port access is not restricted in any way.
                  
The userport.sys driver was written by Tomas Franzon and
is available for free together with the C sources.

How to use userport.sys:
1. Copy UserPort.sys to %windir%\system32\drivers 
2. Run UserPort.exe to set the port range you wish to open.
   Standard LPT addresses are:
   LPT1: $378 ($378 - $37A)
   LPT2: $278 ($278 - $27A)
   LPT3: $3BC ($3BC - $3BE)
3. Push "Start" button
4. Now You can run TwinAVR
5. If You like You can now push "Stop" in UserPort.exe

The only drawback is that you are opening a security hole on your
system that could be exploited by malicious programs. Therefore, open
the I/O port range only that you need access to.
The full userport package including sources You will find under:
http://www.embeddedtronics.com/public/Electronics/minidaq/userport/UserPort.zip
and with Google when using the keywords +"userport" +"Tomas Franzon".