Desktop font

Occasionally when starting my PC, my desktop icons are
accompanied by names written in unreadable symbolic text.
When I try to open any programme, it is all written in the
same symbolic text (triangles, question marks, squares
etc).

I can't overcome this and have to wipe my hard drive and
re-load a previously created image.

I've checked for viruses and this does not appear to be
the problem.

Any advice please would be most helpful