If you were just talking about moving files from mac to PC, the issues would be very minor--you might have to adjust a layout or two to adjust for the fact that windows machines need just a little more area to display the same text.
However, there are more issues with converting 5 or 6 files to FMP 10.
Two knowledge base articles worth reading:
I have used filemaker since version 3 or 4 and one database I created in version 3 is now operational in version 10, aleit with extra features.
Just create a copy and use that copy for the upgrade.
If its a simple database, you will be ok.
if I remember correctly, more advanced features (like tables!) got introduced in later versions of filemaker.
To me the first problem is the data recovery.
The iMac name applies to seven or eight models.
All but the first have Firewire ports.
If the iMac starts and produces video - a blue or grey screen with a flashing question marked folder,
you should connect the iMac to another Mac with a Firewire port and start it with the T key down.
This is called Firewire Target Mode and may allow data to be recovered from the drive.
If the drive is damaged or failing this will not work.
Booting the iMac with an OSX CD DVD will allow Disk Utility to be run on the internal drive.
IF Disk Utility sees the drive, note the bottom of Drive Utility window that reports the SMART status.
IF the SMART status is Failing or Failed, more intense procedures are required
These would intail using DiskWarrior or Data Rescue or moving the drive into another Mac.
IF the failure is mechanical or electronic rather than directory structure, a data recovery service would be necessary.
NOTE there is a PRAM battery in Macs, if the battery fails (3.6v lithium) some models refuse to boot.