Different Font Sizes on different PC's

Please help. I've sent out our companies software today to a customer to start using. It contains a page with several listviews and charts. On my PC, the listviews appearance is perfect. Tahoma font 10, all columns spaced evenly, etc. However, on the customers PC, its all big and chunky, and incorrectly placed. To further confuse things, there's actually 2 listviews on the form, and on the customers screen, 1 appears and looks perfectly fine, the other suffering the chunkly (what looks like 16pt) fonts. My question is, if a user has weird themes installed in XP. Like strange colours and fonts, can this affect your application? I know, when you assign colours, you can choose a system colour like "Window Text", or define your own colour. But is this the same for fonts? I've looked at both listviews and cant see anything different in the appearance properties. However, for the font, they do both actually say "Tahoma 10World". Anyone know what the 'World' is? Any response gratefully appreciated