Constant Disk Activity slowing things down

Hi I have a Dell Inspiron with 4GB of Memory, 466GB Hard Drive, twin Core Processor and 100MB Internet Speed.

I keep getting lots of disk activity slowing down anything that's running or trying load things.

If I get an email with a WORD attachment (I use Thunderbird) and want to open the document, it takes ages to load WORD, sometime I get that Thunderbird is Not Responding, while I head the drive going crazy.

Also loading Internet Explorer takes ages much longer than Chrome or Firefox. 

Any help you can give me would be appreciated.

Let me know if you need any more information.

how much free space is left on your 500GB hard drive? a full drive can cause the problems your experiencing.

I have over 300GB Free Space.

Something I forgot to add was, I'm running Windows 8.1.

ok see if this helps :- http://www.tomshardware.co.uk/answers/id-2045646/windows-100-disk-usage.html