Welcome to MobileRead, rajendra8!
I haven't heard of this particular issue, but there may be someone else around who has. The only thing I can think to suggest off-hand is to reinstall the software.
You
can delete files manually from the Sony Library Software through Windows Explorer. The file locations can be seen by opening a book in the Library Software itself, and clicking on the little i icon toward the upper right of the window.
Usually they're in someplace similar to: C:\Documents and Settings\
UserName\My Documents\My Books\eBook Library.
You can't really add fonts to the PRS devices, but you
can change the existing ones to other fonts. Unfortunately it's rather an involved process -- if you'll search the forum for "PRS500 fonts" that will probably point you in the right direction.
One other minor detail: the PRS500 is named the "Reader" (no "E").
The reason I mention this is that there is something named "eReader," but it's an ebook
format, and that format is
not compatible with the Sony Readers. We've had folks who bought eReader format ebooks and found out the hard way that they don't work on their Sony Readers, so we want to try not to add to that confusion.
I hope that helps.