It might be the hidden partition which stores the backup Image of Windows.
If you have Windows 7 or 8 then the computer is eligible to upgrade to W10 for free when that comes out. I suggest considering that.
When I upgraded my dad's Vista laptop (2GB of system memory, slow 250GB HDD) it was ridiculously slow. After the upgrade it was incredibly fast.
Seriously, it took over SEVEN MINUTES to boot in Vista. Probably a glitch but I couldn't fix it. Booting to Windows 8 was 30 seconds!