Assuming that the SP3 update...can counteract whatever is amiss...is not necessarily true.
SP3 is designed to be a security update, not a repair tool or device. Whatever defects a system has before it is installed...are likely to continue, if they have no root in the XP install itself.
So even though you say that you attempted booting from a Windows disk...that is an incorrect interpretation, since Windows means XP, in your case.
I don't know much about laptops, so the best advice I can suggest is that you review the repair procedures available to you courtesy of HP...and try those (since you do not have an XP install CD).http://h10025.www1.hp.com/ewfrf/wc/documen...product=1842155
One alternative would be to borrow or burn a copy of an XP install CD...and then attempt to use that to repair your HP system via the Recovery Console.
One alternative would be to download an .iso file containing the Recovery Console...burn that to bootable CD...and then use that to attempt repairs.
Download Recovery Console, Bootable ISO - http://www.webtree.ca/windowsxp/repair_xp.htm
The issue of malware will probably remain unresolved by any of these actions and will need to be addressed if/once the system boots.