Word/Excel docs slow to open

I have some users running Windows 2K and Office XP. Some Word and/or Excel documents they receive via email (Outlook)are very slow to open, even small files. Yet not all Word and Excel files behave in this manner. Some open with no problem even though it may be a larger file than the one causing the problem. Once the file has been opened, you can close and it will re-open fine.
Anyone experienced this?
Thanks.
Ed