Big black boxes are showing up, why?

I'm using rapidweaver on my mac to create my website.

I’ve noticed that although in rapidweaver itself, and in Firefox on a mac, everything looks fine, however, when I go to check it in Internet Explorer 8 via parallels (version 6.01192), there are huge black boxes coming up which are not supposed to be there. (see attached)

I've asked others who've said that when they look using an actual pc they DON"T see these lines, but my mac parallels pc IS seeing them, But only when each page first loads, as soon as I resize the screen the black is gone...

Not sure how to fix it, not sure if its a rapidweaver issue or a parallels issue.
Any advice greatfully received.
Gabrielle