HDD is hard disk drive, the amount of physical storage in your computer. I
suspect 256(MB) is the amount of RAM.
Virtual memory uses space on the HDD, even if you had Windows set to manage
it, if space was low on your hard drive you could still get the warning.
Go to My Computer, and right click on your C: drive and go to properties,
this will display a pie chart showing the amount of free space, it should be
at least several GBs.