Q: Start Up Disk Almost Full

So this "your start up disk is almost full" warning has popped up once again. The iMac I use is a Mac OS X Lion 10.7.5
Mine is one of five accounts ( including the guest account ) on this mac. I do not have the administrator account.

My problem arises because, till now, I have had a tremendous amount of space on this Mac until a bunch of stuff labelled

"other" popped up out of no where. "Other" stuff takes up a whopping 412.95 GB. Until this "other" stuff started, I had not
had any problems with this mac.

This is how the memory space is being used on the iMac:

Audio: 1.76 GB

Movies: 58.49 GB

Photos: 3.15 GB

Apps: 20.22 GB

Backup: Zero KB

Other: 412.95 GB

2.69 GB out of 499.25 GB remains free and I have absolutely no clue as to how this "other" stuff managed to nab so much
of space on my iMac. If anyone is able to help me out with this problem, I'd greatly appreciate it.

These are my iMac's specs, if needed.

iMac

21.5-inch, Mid 2011

Processor  2.5 GHz Intel Core i5
Memory  4 GB 1333 MHz DDR3
Graphics  AMD Radeon HD 6750M 512 MB
Software  Mac OS X Lion 10.7.5 (11G63)