To address the problem of can you use an external hard disc as a boot disc, I guess would come down to whether or not the BIOS supports it.

First, I think you're confusing memory and drive space, and megabytes and gigabytes, making your explanation of your problem difficult to follow. Memory is made up the RAM chips on your motherboard, expanded by virtual memory on your hard drive. The contents of your PC's memory disappears when you turn the PC off. It doesn't need to be defragged. 512MB (half a GB) of RAM is low by today's standards, but not unheard of. 1GB or 2 GB are more common.

The hard drive is your PC's primary storage. It's where your programs and data reside, and its contents remain until you erase them or a disaster strikes. Hard drives in the hundreds of GBs are common, these days. I suspect that external hard drive is 120GB, not MB.

In answer to your original question, most PCs these days can boot from a USB devices, such as a flash drive or an external hard drive, but not all operating systems support this. Windows doesn't. You can boot DOS or Linux from an external drive, but not Windows.

I recommend one of two approaches:

1. Use EasyCleaner's Space Usage tool (EasyCleaner is free and doesnt have to be installed) to see what's taking up all the space on your hard drive. If you're like most users these days, it's media (photos, music, video). Move that to your external drive.
2. Buy and install a larger internal hard drive. They're cheap, these days.

Lincoln