Make sure you have the boot drive into IDE channel 0 (or 1, however your board is labled), and also check the jumpers again... Some drives have a setting for single drive, and setting to primary implys a slave, which can make the drive not boot. FAILING THAT, however, you may have to run the XP repair install option (works rather well, actually). It basically reinstalls any system files that are corrupt or missing. You'll lose any customized options, such as desktop, contact list, themes, etc... but your data will be intact if you've installed anything. Also, you'll have to grab any service packs you may have installed over again. Not to bad though, no need to reformat.
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