PC freezes when opening Outlook Express?

I have a win98 machine that freezes everytime Outlook Express is opened. No mouse inputs, no keyboard inputs. I have to reboot the computer,(sometimes Ctrl, Alt, Delete works…other times I have to use the power button). I can’t tell if it’s OE 5.xx or 6.0. If it is 6.0, and I uninstall it and revert back to 5.xx, will I loose my address book and saved emails? Any ideas on what is causing the PC to freeze up?