u can create user defined functions by saving the file 
as .vbs..just open notepad and write as below and save the 
file with the ext .vbs

public function test
x=6
For i= 1 to x-1
	x=i*x
Next
	msgbox x
end function
then in QTP goto
File>settings>resources tab..add the .vbs file u created..
and then write call test.....in ur editor.

thanks 
chandu

Creating User defined Functions:
Open QTP and Application --> type function header along 
with Unique function name and arguments --> Record 
repatable operations in application as function body --> 
follow above navigation to create more then one functions --
> Save that functions in a file using 3rd party s/w with 
Extension .vbs( Copy the script, genarated in QTP and paste 
in Notepad, MS.Word etc.) --> now goto Resource menu in 
QTP --> object Repository --> file menu --> Export Local 
Objects --> Enter file name with Extension .tsr --> click 
Save.

Calling User defined Functions:
In QTP --> type Useer defined function name along with 
argument values --> file menu --> settings --> Resources 
tab --> click + icon -->browse the path of the file --> 
click apply --> click OK --> Resources menu in QTP --> 
Associate Repositories --> Click + icon --> browse the path 
of references file --> select current action --> click OK.


Example:( Creating User defined Function )
Public function login(x,y)
Dialog ("Login").winedit("User name").set x
Dialog ("Login").winedit("Password").setsecure  
crypt.encrypt (y)
dialog ("Login").winbutton("OK").Click
End Function

( Calling User defined Function in a test )
login "prashanth","testing"

U can use " Function Definition Generator " to create a new
user defined functions