Please Help!

Ok my mothers computer which is a HP Pavilion 754 running Windows XP currently will not boot up, and she has called the HP tech support and they checked her BIO’s and hardware reseated the RAM and everything is fine, hardware bios everything. Now the thing is.. she has not reformatted her computer in over 3+ years and had some Trojans on there, i kept urging her to Reformatt and of course never did. so i would like anyone with any information to please help and contact me via E-mail , and Thank you all whom take the time to read and (if) reply to this post. Thank you so much.