What's the HDD spec.
If its IDE 5200 with a very small / non existent cache move your OS off this immediately. Just use this for backups / archives.
Get a decent SATA disk with tha largest cache you can afford -- minimum 7200 RPM. (If you can affford it SCSI or SSD is usually the best for performance but these rae more expensive).
Set your Bios to use SATA as well (enable AHCI) instead of IDE. - You'll need to "repair" or re-install Windows if you change this setting BTW.
A lot of Bioses have IDE set as default. Yours probably will as well if you upgraded from Windows XP as a normal Windows XP installation doesn't have SATA drivers included. You have to "Slipstream" these into a windows XP install disk.