All these programs require your discriminator scanner output.
There are lots of different programs on the internet, that do pretty much the same thing.
Like any other software there is good and bad programs around.
This page is based on my personal experience.

PDW This is the best program to date as development has been continued through to Windows XP. Requires an interface or sound card.
PDW is freeware!! Download here Special thanks to Peter Hunt for his programming work!!

WINFLEX V1.0 This one of the  best programs I've found. Download here
Requires Windows 95/98/98se and the  interface for this to work.  THIS SOFTWARE WILL NOT WORK WITH WINDOWS XP!

POC32 V2.06 Works using your sound card It's a great shareware program, this requires no hardware to be built unless you want to decode more than two signals at once.
The down side is it doesn't decode all  the messages with the soundcard , Flex not supported.
Requires Windows 95/98/98se and the  interface  or a sound card for this to work.  Download here

Pocflex similar to Winflex this is the Dos version
Requires Dos and the interface   Download here

PD204 Works Ok. Good program to leave running forever on that old 386.
PD runs on an IBM PC or equivalent, anything from an 80286 upwards. It requires 512K of conventional memory and a small amount of hard disk PD runs from DOS, running from Windows is not recommended. interface Required for this to work Download here

Pocsag/Flex decoder REV 0.004 Dos based very similar to PD204 seems to work OK
Download here