If you've got the WIN98 bootup menu activated (with a timeout larger than 0 seconds if you use TweakUI), then you can select safe mode from that boot menu. Note that this is the menu after you've already selected wich OS to load. It's the same boot menu that lets you boot into "Command Prompt Only", etc.
If you don't have that menu, then holding down <CTRL> during the WIN98 boot sequence should bring up the boot menu.
Good Luck.