Have you checked all of your keys? It may be as simple as one of them is stuck and that's why it's starting to appear on your screen.
Have you got another keyboard to try on your PC? If this corrects the problem it's a faulty keyboard so you'll need to buy a new one.
If your still having the problem submit a hijackthis log here
and find out if any malware is causing the problem.How to submit a hijackthis logDownload Hijackthis
Try running the following from safe mode (Getting to safe-mode) Sysclean
you'll also need the virus template file from here lpt***.zip
remember to extract the contents of the zip file into the same folder as Sysclean.com
If your good with the command line also try Sophos Command Line scanner
this command will scan all of your hdd's SAV32CLI.EXE -F -di -remove -dn -mbr -all -zip -p=avscanlog.txt and give you a log file to review afterwards.Also try installing and running A2 Free and Ewido
I'd also run Spybot(Spybot Tutorial)
and AdawareIf your using Win2K/XP run adaware/spybot from "safe mode with command prompt"At the C:\ prompt type the following:-
C:\progra~1\spybot~1\spybotsd.exe /autocheck /autofix