A client of ours has a windows 98 pc and runs an old dos based accounting system. There are different modules belonging to the accounts system which are run by selecting the relevant icon from the desktop. every couple of weeks or so when the user runs a module the dos window displays a load of garbage characters. You can still just about make out the dos programs interface but the program is pretty unusable.
If I delete the shortcut on the desktop to that particular module and recreate the shortcut it works fine for a few days and then another dos program shortcut will start acting up.
There are no viruses on the PC. Ansi.sys is loaded. The same program is run from other win 98 pcs with no problems at all. All other icons and programs are fine.