You need to boot from the XP CD to reinstall the operating system. Since you tried installing 98 so I think that the restore partition won't be there anymore.
Still there is no harm in trying that. Restart the system and hold the CTRL key down and when the Dell logo screen appears, tap the F 11 key thrice, slowly. If the restore partition is still intact then you will get the Dell PC restore screen, select the option, restore my system to original factory settings and click next, mouse won't work so use the keyboard to select the options. Then select the confirm button an.d press enter. The restore process would start.
If the restore partition is wiped out then you need to boot from the XP CD, the steps are as follows:
Restart the system and tap the F 12 key when the Dell logo screen appears, it will take you the boot device menu. Select the IDE CD-ROM drive option ( it can be DVD or CD -ROM so don't get confused, there would be only one optical drive option so select that .
Then place the XP CD in the optical drive and press enter, soon you will get a message saying "Press any key to boot from the CD". Don't waste even a single second and press the enter key.
You will get a screen that will say set up is loading files. Once its over then you will get three options, the following link has the step by step instructions with screen shots from how to take on from there.
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After you have installed theoperating system make sure that you install all the drivers. If you don't have the drivers CD then give me the service tag of your notebook computer and I will give you the link for all the drivers and the sequence in which you need to install and apart from that I will also give you the link for the necessary applications that you may need.
Good Luck!!!