Try going to the Library (in LW) and clicking the Incompletes folder, then browsing the files until you find an incomplete that looks like the one that was at 99% (so if the complete file was 100MB, then that incomplete would be 99MB and so on. duh ). If you find it, highlight the file and click resume. LW should try and resume the download from 99% (you can at that point try searching again for the file through the search function in order to help LW come up with some sources).
If, now, you can't seem to find a file that looks like 99%, then it's time to consider that the file has been deleted from the download.dat list and thus LW doesn't recognize it as an incomplete anymore. In that case... I'm sorry but I can't help you.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

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