Treadmill causes nearby electronics glitch
You have EMI issues, as you have deduced.
This can be radiated or conducted via wiring.
The problem may be poor EMI suppression in the treadmill, or poor PC system EI rejection or, quite possibly, some mix.

Without capable test equipment it is difficult to quantify the magnitude and bands of noise transmission and the location and types of noise sensitivity BUT, lets just fix it instead :-).

If you can operate TM (treadmill) and PC (PC system components) from two independent power supply phases it MAY help.  But, only maybe.
If operating from a single mains outlet.
Use two "plug boxes".

Throughout the following tests I  repeatedly say “report the results”.
 the intention is to identify changes in noise level which may be made by changes in configuration as various combinations are tested.  Failure  to record results  will lead to  uncertainty as to what did and didn't work.

Operate all PC equipment monitor etc with mains power cords going directly from a single plug box to the equipment.
Operate the treadmill from a separate plug box.
Report the results.

You mentioned having an EMI filter.
Plug the treadmill blood box into the wall outlet and then plug the PC plug box into the treadmill plug box via the EMI filter or, if available, directly into a separate wall socket.
If you have a second EMI filter that is capable of handling the treadmill peak current use that to connect between the treadmill and the wall outlet.
Try operating the system as described and report the results.  

Next if you are able to borrow a UPS (uninterruptible power supply)  capable of running the PC or the whole PC system, operate the PC via the UPS and report the results. 

Next, pull the UPS power supply plug out from the mains so the PC system is running on the UPS  and report that results.
You would hope that if the signal is conducted via the mains,  then running  the PCfrom a UPS alone will produce a superior result.

Next without using the UPS borrow  an isolating transformer (110 to 110 or 230 to 230 volts AC depending on your local power supply voltage) and with enough power capacity to run the PC  or the whole PC system via this transformer and report the results.

Also try running the EMI filter in series with the supply to the Transformer.

Obtain a multimeter with an ohms range  set it to low-ohms and with the equipment all turned off but plugged in to the plug box.  pull the power plug from the plug box out of the wall  socket and Measure  the resistance between  the plug box ground pin and earthed portions of each Item of equipment, to ensure that there is a low ohms connection between for instance the PC ground and printer ground or  PC and keyboard metal and so on. It may be that the PC metal is not grounded.  If any of equipment is not grounded make it so.  if uncertain about grounding ungrounded equipment seek local professional help. 
Report the result.

At this stage report back with  individual and overall results and we will see if you  are getting somewhere or if a different approach seems warranted