how do i open a pif document that i receive by e-mail, my computer
won't let me open any emails with documents attached?

a pif file is a program information file that gives information to a
certain program to start up . most likely it is a virus or a worm and
you should delete it asap , if you are really curious what is in it ,
save it to your disk and use notepad to open it (but you are doing
this at your own risk)

This is the precise reason your computer won't let you open e-mail
attachments.  Viruses are spread by trusting people who open any
attachment that comes their way.  Unless the person who sent you the
pif file has spoken to you about it and has a valid reason for sending
a .pif file (which is almost none) and is someone you trust, I would
delete it immediately.  Note I say they should have spoken to you and
not just be someone you trust, because viruses are now likely to come
from someone we know who made the same mistake of opening an
attachment and launched a virus without knowing it.

First let me say that I agree that the PIF file is very likely a virus
so do not attempt to open it.

However, if you work with some IBM vector graphic soft software you
might also be using  files with the PIF extension (very, very unlikely
since you didn?t mention any  unique applications.)

You mentioned that you could not open any email attachments.   I
suspect you are using outlook express, which by default blocks ALL
attachments regardless of  the attachment being  infected with a virus
or not.

You can change the settings in outlook express to allow viewing of the
attachments. However, BEFORE doing the following  make sure your
antivirus application is up to date with ALL the latest updates.  And
Only download the attachments that you were expecting from someone,
NEVER open an attachment just to be curious about what it is. The old
saying curiously killed the cat holds true meaning when it comes to
computers.

To allow viewing of attachments in outlook express 
click on  tools 
select options
click on the security tab.
Then UNCHECK the line do not allow attachments to be saved or opened
that could be potentially a virus.

Unfortunately when that line is checked ALL attachments are blocked.

However many times that is a good thing for these days unfortunately
most attachments  very likely contain a virus.  So only open files
which you are specifically expecting from someone and even then do a
manual update on your antivirus software and let your antivirus
program scan them BEFORE you look at the attachment.

Hope this answers everything for you.