Whether this is true or not, I'd start planning an upgrade.
Filemaker 5 is really, really old. The day is just around the corner, if you haven't reached it already, where you won't be able to use it without using an old and unsupported OS version.
We are looking into either upgrading or finding another program all together within a year. Do you think that will be soon enouogh?
I don't see why not--assuming you can solve your current issues.
If you want to evaluate how much it will take to convert your files to FMP 10, you can download the free trial version and experiment. (I've converted my 5.5 files to fmp 10 so it can be done, but there will likely be issues to be resolved before the converted files perform correctly.)
If you are using filemaker server, check out the system requirements carefully, you might have to upgrade both server OS and server hardware.
BTW we are running FMP 5.5 on windows xp sp3 without any issues. Our big "roll out" to FMP 10 is scheduled for this weekend.