Yes, you can drill into uPVC windows - for example you may want to fit blinds to your windows.  However, doing so may compromise your guarantee. If you are unsure, then it is always best to speak to a qualified installer.

There are a number of things that you should consider when drilling into a UPVC window frame.  The most important of these is to ensure that you are not drilling close to the double-glazing panel.  If your drill comes into contact with glass, it can shatter the panel which will need to be replaced.

Similarly, you should avoid using a hammer action drill when drilling into UPVC.  The frame is tightly fitted into place, and the vibration from a hammer action drill might cause some of the fastenings or the locking system in the frame to become misaligned.

When drilling into any window frame, you should always be as careful as possible.  Avoid using the drill at full power, and prior to starting, you should make use of a metal detecting tool to ensure that you do not drill into an area where there are locking system or hinge components.

Your installer will be able to provide you with advice about where you can and can't drill into a window frame, but if you are uncertain about the process, it is always wise to contact a professional to make any changes to your installation.