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Is that the only noticeable effect? I havent heard of a virus that does that, but that doesnt mean there isnt one. Perhaps its just their keyboard freaking out (if its wireless)? If McAfee doesnt catch anything, you should try out http://housecall.trendmicro.com
What do they have for an internet connection? Do they have their antivirus/firewall set up to scan everything? You can have the best AV software in the world, but if you dont have it configured properly, its useless. You might want to have the neighbor hit up google.com or something, and search for 'upper case viruses', or something. Even that, they should go to the McAfee site, and see if they have any warnings for such virii (?). Do you know if they run any spyware/adware software?
download and run AVG virus software.
Mcaffee isn't as good as it used to be.
"The teenager in that house does much pier-to-pier and IM"
it it sounds like it might not be a virus, but a script the teen installed, like AOL powertools or something for a IRC client