Freezing computer

Hello,
We have a station, HP Pavilion 8750, that constantly freezes ? first thing in the morning. I have two identical machines & the other doesn?t have this problem. I?ve switched hard drives & the problem remained with the hardware. HP wants me to reformat the hard drive, but I do not think it?s the software. Should I just return the machine ? my warranty is good until August 2001.
Thanks,