Inspiron 1300 4 months old

Windows XP Home

512Mb RAM

80Gb Disk

 

For a few months, sometimes the characters which appear on the document are not what I type on the keyboard.  This only happens on one application at a time (not all the applications which are running). The other applications are not affected.  To resolve the problem, I quit the application and restart it.  If an application eg Word has three documents open, I need to quit all three.  This is not related to any particular application, it has happened to Word, Wxcel, IE, Notepad and all other applications I run.

Any idea what could be plaguing this system?  It's not hardware related otherwise it would happen to all applications at once.

Thank you for your help.