There are various opinions about the best way to clean uPVC windows. It is hard to know for certain what the best way to clean uPVC is. The best advice is NOT to use scouring pads. Abrasive cleaners will scratch the uPVC. Unlike wood it is not a simple matter to sand down a scratch and reapply a varnish. A series of scratches on uPVC is difficult to repair.
For this reason a soft cloth or a baby wipe is recommended to use for cleaning uPVC.
The main question is what type of cleaning product to use. One source of good information is the supplier of the uPVC window. They will normally be able to recommend a suitable cleaning product.
Washing up liquid with water is one commonly used cleaning product. Another traditional cleaning method is to use a mixture of vinegar and water to clean the uPVC.
Other cleaning products that are recommended by housewives are Jif, Astonish, Flash Mark and Stain Eraser. This link has a good list of reviews of uPVC cleaning products.
Finally, there is a very good chance one of your neighbours has uPVC windows. Ask them what they use.
Reminder - This post is about cleaning the uPVC frame AND NOT THE WINDOW