Java Programming

I have been running code using a Graphics object for over 2 years now without any problems. I havent used any Graphics2d objects and I use double buffering to paint the objects to the screen.
I use Netbeans and the game can be launched from the web site using webstart.
The game gets soundfiles when needed from the web site too.
For the last week there have been ugly texturedsquares moving all over the game screens. This only happens on my development machine.
If I copy the dist folder with no changes to another pc it runs without any problems. Similarly when the game is launched with webstart from the web site there are no problems on my other machines.
The sprites look like they belong to some other game and I have at no time written code like this in any of my applications. There are no viruses or malware on the machine and the caches have been emptied, old java versions uninstalled and Netbeans has been updated to latest version.
The problems started when I visited an IRC channel and I am sure that somehow, something on my pc was changed. I can find no evidence of this and my code has not changed.
I use classes that paint themselves using image files mostly but these sprites look like they are being drawn using the Graphics2D class.
The images seen to appear randomly and they change colour and pattern. They dont appear where a repaint is needed and they are larger than my own sprites,
Does anyone know how I can detect this problem's source?
I have been writing the game for over 3 years now and I can see no reason why this is happening.
I dont want to have to format my development machine but it looks like I might have to.
I would be really grateful for any help with this!