Soon after I bought my car the first time I cleaned the windscreen I got this foul egg smelling smell come through the vents - I can only presume the car had been sitting for a while and water got stagnent.
One option was to remove the wheel/arch/bottle but I just did not have the time. Then I read someone reccomending using a calgon tablet. I crushed the tablet up into small pieces and mixed it with hot water. I poured it in left it for 30 mintues and drained - smell was gone
Tip never mix different brands of screenwash, stick to one type