Does windowing affect Parseval's theorem?
Parseval's theorem will hold, but take into account that your signal in the time domain will no longer be $x[n]$. Namely, if you have that

$$\sum_{n=0}^{N-1} \Big| x[n] \Big|^2  = \frac{1}{N}  \sum_{k=0}^{N-1} \Big| X[k] \Big|^2$$

then, if you window the signal $x[n]$ with a window $w[n]$, your signal will now be $\hat{x}[n]=x[n]w[n]$, and the theorem will now return the following equality:

$$\sum_{n=0}^{N-1} \Big| \hat{x}[n] \Big|^2  = \frac{1}{N}  \sum_{k=0}^{N-1} \Big| \hat{X}[k] \Big|^2$$

where $\hat{X}[k]$ is the DFT of $\hat{x}[n]$ (and obviously differs from $X[k]$).