C:\Windows\Installer file is 94.3GB in size, Help!?

Hello this is my first time on here.

Basically my family own an Acer Aspire computer which is full to the brim. Looking at the C: drive it says; 3GB free of 139GB. However we don't even have that many files or photos. I did some investigation and using 'TreeSize Free' I have discovered that:
The 'Windows' Folder is a size of 119.9GB!
I then found a folder inside windows called 'Installer' which is a size of 94.3GB!

This can't be the right size of the folder, we cant run anything on the computer. I managed to get the 3GB of space free by moving everything over to an external hard drive.

Please can anyone help me solve this?

It's windows vista home premium 32bit with 3GB ram. Thanks in advance to any replies.

Please post a TreeSize screen shot

This should be the safest first step:
Start Disk Cleanup

MilesAhead said:

This should be the safest first step:
Start Disk Cleanup

Click to expand...

Just ran a disk cleanup and it has not changed the size. :/

CCleaner is worth a try. If that makes no significant difference
I would try a malware scan with Malwarebytes antimalware free.

Already have CCleaner installed and no luck, Removed some malware that was on the computer but still no change.
It also does not boot up Windows Defender, I have tried going into System and checking it was on 'run automaticaly' and have also started and stopped the application yet it still wont boot up.
It also will not download and install updates and gets stuck on the 'installing updates' screen.

"Installer" shouldn't be there. So do you have any strange entries in your Program list in Control Panel? Or, does your ISP or PC makers have one of their own proprietary computer "management" console installed?
Those are often very top heavy and I usually recommend people get rid of them.

I would run a few anti-malware tools if I were you. Malwarebytes Free for instance, and maybe enlist the help of a good anti-malware forum such as BleepingComputer to analyse a Hijackthis log to see if there is anything strange going on in that machine.

Hijackthis can be downloaded here: HiJackThis | Free software downloads at SourceForge.net

BleepingComputer log analysis forum: Virus, Trojan, Spyware, and Malware Removal Logs Forum - BleepingComputer.com