When I reach the point in the installation when I have to declare the partitions, Disk Druid displays an error message and shows only one of my hard disks.
When running fdisk with the other hard disk, it displays the message "The number of cylinders for this disk is set to 4998. There is nothing wrong with that, but this is larger than 1024, and can cause problems with partitioning software."
How can I solve this problem without losing data (I have Windows98 installed) ?

It's just a warning for people using old versions of Lilo or installing on old systems. If you're installing on a system less than a pentium 200, your bios might not boot disk partistions over a certain limit (1024). WIth windows, this wasn't a problem, as windows was the only option. When Linux came around, it was a hit and miss thing. For 90% of people, it wasn't a problem, as LBA was enabled, but if your bios didn't have this option, you'd have to work around it. Without any guarantee, if your system is newer than 1996, you're safe, and can ignore this warning. If it's older, you can check the BIOS and see if you have a LBA option. If so, enable it, and you're good to go.

The strange thing is that I have two identical hard disks and the Linux fdisk displays this message only with the one that causes an error with Disk Druid.
Disk Druid says there is a problem with the partition table on that disk and offers me the choice of erasing the partition table or ignoring the error ; I chose ignore, and only the error-free disk was accessible for defining partitions...