Hi Edward.
Not in my current window cleaning life, but when I worked for Bosch Power Tools in South Africa we did it in the very late 80's after it had changed from BS5750.
I know things must be different now, but the process was a whole lot of business accepted documented procedures that the company followed in all aspects of their business dealings. In our case, it began with importation of goods, warehousing, sales and invoicing procedures, goods dispatch and delivery as well as all workshop after sales service procedures.
This was adopted by Bosch as a worldwide standard in their organisation, and was a way to get each branch in each country to run to the same procedures.
So theoretically, I could have walked into any after sales service branch in any country and begun work as the procedures were the same throughout the world.
So from Bosch's point it was a good thing. They are also a massive company and other big companies would take comfort knowing that they work to this creditation.
With that in mind, I would feel that nothing would be gained from an ISO9000 creditation as a small window cleaning business even if you had several employees. I bet one of your large customers will only employ other business that have ISO9000. They won't deal with you if you haven't got it. But the paper trail needed to support that creditation is unbelievable (or it was). We were always having spot checks on our day to day processes to ensure we we working to the 'book,' all in preparation for the big ISO audit that could strike at anytime.
We have walked away from a large job that required us to be VAT registered. We aren't and I can't see any benefit for me at my age to become VAT registered for this one job.
I would think the same would apply to ISO 9000 IMO.
Spruce