but you don't appear to state what O/S you are using. If it is Windows 98/SE you can use your boot disk/"start up disk" and use the Fdisk facility.
Type fdisk at the A> prompt, and then in fdisk use option 4, which will enable you to view the partitions on your drive. Presumably you only have the one hard drive installed?
Could be that your hard drive is on it's way out and most certainly if you can back up the files you can gain access to, it would be wise to do so.