Coleen:

Paraformaldehyde is a solid that must be depolymerized to be put into 
solution.  When it is depolymerized it is then formaldehyde, so although the 
solution is referred to as paraformaldehyde, it is in reality formaldehyde.  
The difference is that formaldehyde solutions prepared from paraformaldehyde 
are purer in that they do not contain methanol.  Methanol is added to 
formaldehyde solutions to keep it from polymerizing into paraformaldehyde and 
precipitating out of solution.  The methanol has been thought to give 
inferior results for electron microscopy, so solutions prepared from 
paraformaldehyde are preferred for that purpose, but I doubt that you will 
see any difference at the light level.  In fact some of the work that we did 
at the EM level showed practically no difference.  You are getting 
formaldehyde fixation in either case, with questional effects of the methanol 
present in commercial formaldehyde (37-40%) solutions.

Hope this helps.

Freida Carson