Did you get the comp and this happened, or have you been able to use it before then this started to happen?
To me it sounds like your hardrive is going out. Maybe its not fully connected anymore, has a bad bootsector/os installation, or is dieing.
Ive seen this problem also when a laptop tries to boot from lan, so going into the bios and making sure your hard drive is high or first in the boot list could do it.