Vista has a habit of keeping loads of bits and pieces, 15% of the space will be used for System Restore Points, it also keeps copies of all the update files meaning the Windows folder often reaches 13 or 14Gb on its own.
From your two partitions I would suggest you have an 80Gb drive, 13Gb Acer will have reserved for the hidden partition, if you made the Acer recommended back up discs you can actually delete this partition and re-use the space (just don't loose the CDs)
You can use the disk clean up options sugested buy Derek C, this will clear a lot of the rubbish, download this small file and run once a week to clear out the basic rubbish that builds up.
http://majorgeeks.com/ATF_Cleaner_d4949.html
you can reduce the size of the system restore points in Vista but have to go through the command line to do it, so if you are feeling brave have a go at this.
To Change the size - right click and open the command prompt found in Start > Programs > accessories) and run as Administrator. Then use the following command to change the size of System Restore in Windows Vista:
vssadmin resize shadowstorage /on=c: /for=c: /maxsize=4GB
This would change the allocated size of System Restore on drive C: for drive C: to 4 gigabytes which is more than enough. Then close the open window and reboot.