Are you saying that if you type in another application, say MS Word, Notepad etc, then all is fine, but when you type in your email client, there appears to be a lag between you pressing keys and them appearing on the screen?
What is your email client (Outlook, Outlook Express, Thunderbird etc...).
Struggling to think of a reason for this except that maybe you are low on system resources; basically you need to clean up your system and/or get some more memory (which is very cheap at the moment, I just bought 1GB laptop memory for �20).
Also, later versions of MS Office (after version2000) use Word for new emails (assuming you have selected that option, which is the default), but this should be no different to typing in Word.
Paul