Hello and welcome to the forums, I need to ask a few questions here first.

Has the damp patch only appeared since the new window was fitted?

How close to the new window is the damp?

Does the damp start from floor level or at a higher level?

What month and year were the windows fitted?

What month did the damp first show?

Describe the colour and size of the damp?

Is any object like furniture normally sited against the wall where damp is?

Sorry for all the questions, but I need some facts before suggesting an answer.

Again thank you.

Pro