To answer your questions, we need a little more information. Normally when you create a true dual-boot system, you have the option at startup to choose from one of the Operating Systems on your PC before the Windows splash screen appears. If you're not seeing that, then a few things could be causing this based on Windows settings or the way and manner the Operating Systems were installed...
1. Did you install W98 on one Hard drive (HDD), then overwrite it by updating to XP???
2. Did you install W98 on one HDD on one PC and XP on the HDD on another and then decided to install both HDDs in one PC???
3. Do you have "Display list of operating systems for xxx seconds" checked in Startup and Recovery? I don't use XP, but in W2K you can find this setting in System Properties on the Advanced tab, but it may be the same in XP...
4. If you are using two HDDs, are they both installed correctly? i.e. Can you view all the partitions (drives) on both HDDs???

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