In short no. You could download the Win98 boot disk and format thats no problem, but without a Windows disk to reinstall you've had it.
Are you sure you need to format? If the computer came with Windows preinstalled there may be a partition of the hard drive with windows on it from which you could reinstall, but not if you format it I hasten to add because the format would destroy everything.
Why reformat? If you consider that you have sensitive material onboard then simply delete it. Yes I konw the stories about what is deleted has not gone, but it rather depends who you sell the machine to doesn't it? It still takes a fair bit of IT acumen to do the dirty deed But investigate the partion aspect there may well be way out! Or if it is one of the mainstream brands take it up with them, there may be a way to clean the machine.