If you apply any of the KPT3 filter effects (except for the Interform
filter), Photoshop 7 will lock up solid.
As a temporary (or even a permanent) workround you can use the keyboard to
save the image by doing a SAVE AS ([ctrl]+[shift]+S) or a SAVE A COPY AS
([ctrl]+[alt]+S) and save as either a TIFF or a BMP file. This will free up
Photoshop and you can continue working with your image.
If you are working with a layered image, it is important that you are saving
with the advanced TIFF format turned ON - saving as either a standard TIFF
or a BMP will flatten the saved image. However, the image which you have
open in Photoshop will retain its layers until you finally save it - and
then you can choose from the complete range of formats.