It's not too tricky.

First, use RUN so that the process detaches from the script and runs in a seperate thread.

Now wait a bit and then kill the process using one of the "proc" UDFs.

Here is an untested example, you will need the EnumProcess() UDF.

 Code:
$sProcess="WinDump.exe"

; Start the windump process, use Run to allow script to continue
Run '"'+%COMSPEC%+'" /c \\path\'+$sProcess+' -i 2 -q -t | find /i "length" > c:\text.txt' 

; Wait 5 miniutes...
Sleep 300

; We've done enough, so kill process
$=EnumProcess($sProcess,"true")

Exit 0