Am guessing you want to change the screen resolution on a PC running XP. To do this :
Go to Start Menu
Select Control Panel
Select Appearance and Themes
Select Change the Screen Resolution
You should then see a slider to allow you to change the screen resolution to someting suitable, eg. 1024 x 768 pixels
Click Apply
As I recall, Windows 98 gave a default resolution of 800 x 640 pixels. Sounds like your configuration has changed. You may have to re-install the graphics card driver and the monitor driver because Windows 98 might not recognise modern Plug n Play devices. Once you've got the drivers installed you should then be able to select the correct resolution.
Windows 98 is no longer supported by Microsoft. It might be that these versions of Messenger don't work on 98, check the minimum specification.