Could be anything really... Low on ram, fragmentation of hard-disk, low cache hard-disk (maybe a 2mb one), even Cache of the DVDRW drive, Could be faulty / old IDE CABLE, COuld also be that u didn't install your chipset drivers of the IDE controllers for your specific motherboard, Maybe some applications running thats using lots of CPU or maybe memory, Maybe anti-virus scanning the hell out of your system.
Its a tuff one, and could be a any one of the above.