Marcel
You can use cCleaner to do a more effecttive job of removing redundant files but I am becoming convinced that getting to 25% free disk based on your latest message is not achievable by the methods I have suggested to date. The core of the problem is 44.4 gb audio-video files. You either remove a large part of this collection to removable media like a flash drive or you replace the hard drive with someting like this:
http://www.amazon.com/Serial-ATA-Notebook-Laptop-Inspiron-11z-1110/dp/B003UIXCBS
The price differential between the US and the UK is a bit startling.
A cheaper and simpler alternative would be to a buy a 32 gb memory stick like this example:
You can get 64 gb memory but they are nearly twice the price. The memory stick solution is a much simpler one to implement and it can be retained if you later choose to replace the computer.
Before buying a replacement drive you need to check you have the means to reinstall your system on a new hard drive. Do you still have the Dell Operating System CD and the Dell Drivers and Utilitities CD that came with the computer -see page 85 of the Manual:
You should backup your data files and favourites to external media before replacing the harddrive.
To improve your system performance I would replace Trend with freeware alternatives.
Recommended replacement security products follow:
http://free.avg.com/gb-en/get-basic-protection
http://www.superantispyware.com/download.html
http://www.malwarebytes.org/products/malwarebytes\_free as a support scanner
This is a no cost solution for a home computer user.