the 3gig difference could be down to system restore, your laptop as an old installation of windows will have several restore points taking up space.
the new install of windows will only have one or two taking up a lot less space.
windows xp system restore takes upto 12% (on maximum setting) of the hard drive space for restore points before it removes any, you can alter this to a minimum to save space but the trade off would be less restore points available.
if you want to change the settings, right click My Computer, click properties, click System Restore then choose the hard drive/partition and click settings. move the slider to your desired ammount of space.