Do you have OpenGL enabled? If so, turn it off. Such freezes would then be indicative of transparency blending issues in OpenGL because your graphics card doesn't handle it properly. Of course try the usual after that - newer/ different drivers, settings tweaking in the graphics driver etc.. Additionally, check your region and language settings. Sometimes odd combinations cause "phantom fonts" (which are remapped character ranges derived from of other fonts, required for legacy compatibility in some progrrams) to appear in the font list. When AW tries to access the default font, it wrongly ends up seeing these hidden fonts and acts up.
There was no version of After Effects 7.1.
What version of AE are you running, which version of Windows, and what language is the OS?
Thankyou Mylenium. Turning off Open GL has done the trick.