I had this happen to me yesterday...every row in every spreadsheet (newly
created or existing) had a page break...but on another machine everything
looked and printed as it should.
I discovered the culprit, at least for me, was a printer I had just added
using "Add Printers" in Control Panel. If I selected another printer from my
list the spreadsheet looked and printed as it was supposed to. I deleted the
offending printer from the printer list, then reinstalled it (its drivers)
using the CD that came with it, and everything then worked great. So,
although I can't be sure that this is your problem you may wish to check to
see if (a) the problem happens with every printer you have on your printer
list (if you have multiple choices) and (b) if you delete and then reinstall
your printer drivers.
Good luck.