The space used on your hard drive is data that you have put on it, plus the operating system, plus any programs you have installed and the data files they generate.
Have you Emptied your Trash? When you throw files away they don’t get deleted from the drive until you Empty.
Have you been downloading music or movies or games? The obvious answer is to delete the stuff you have downloaded and no longer need, or move them to external hard drive or DVD-Rs.
Are you using torrents?
Are you automatically subscribed to podcasts?
You can run a program CCleaner which will clean out caches and temporary files www.piriform.com/ccleaner
You can run a program called PCDeCrapifier which will delete unneeded software that the manufacturer shoveled onto the drive. pcdecrapifier.com
Your machine may have some program that is not erasing its temp files properly, or a runaway log file. Try TreeSize (free or paid) or WinDirStat (free) or Directory Report ($25) to help locate which files and directories are oversized.
Clear out your Windows Update cache en.kioskea.net/faq/2471-windows-cleaning-the-update-cache
You can limit the amount of hard drive that Windows System Restore uses for storing Restore Point information:
Win 7 www.mydigitallife.info/how-to-change-windows-7-system-restore-system-protection-disk-space-usage-size-limit/
Win Vista www.mydigitallife.info/change-and-set-vista-shadow-copy-or-system-restore-disk-space-limit-in-explorer-gui/
Decide if you want to disable Hibernate mode www.howtogeek.com/howto/7564/how-to-manage-hibernate-mode-in-windows-7 Definitely do this if you have an SSD drive.
Do you have more than one partition on your drive? You may have a 300 GB drive but if it is divided into a partition of 60 GB as C: and 240 GB as D: drive, you could well be running out of space on your C: drive (your default boot drive) while there is lots of room on D:. Unfortunately, you can’t easily transfer free space from one partition to another. Trying to do so will result in all data from one or both partitions being deleted. You can try backing up all the data from the second partition (D: in this example but it may be different on your machine) and expanding the size of the primary partition C:. Please read up on instructions for your specific version of Windows before attempting this.
Commercial and freeware partition products
Acronis Disk management products: www.acronis.com/?gclid=COOIqKuN_LECFeaDQgodNDkABg
EaseUS Partition Master download.cnet.com/EaseUS-Partition-Master-Home-Edition/3000-2248_4-10863346.html?tag=mncol%3bpop&cdlPid=10982635
WhatSize will find the large files whatsizemac.com
OnyX will allow you to delete temporary files www.macupdate.com/app/mac/11582/onyx
Monolingual will allow you to delete unneeded foreign language resources in OSX 10.4 – 10.6. monolingual.sourceforge.net
Ultimately, if you want to keep the data you have, you may need to get a larger hard drive and clone your existing drive onto it. www.computer-answers.ca/2012/computer-questions/windows-questions/q-how-do-i-get-the-data-off-my-hard-drive-so-i-can-reformat-or-replace-it