Hi,
IM using CCleaner for the first time and i am on the registry section. ive scanned for ussues and a lot have come up so when i click "fix selected issues" it asks me "Do you want to back up changes to the registry?"
Well do i want to do it? what happens if i do or dont do it cheers.

It's perhaps a good idea until you get used to how CCleaner operates.

What it will do if you say yes is make a copy of the reg entries you are deleting in case there is a problem and you want to put them back.

They are stored by default in 'my documents' as a .reg file (it will look like a piece of paper with blue blocks on it)

To put them back just double click the file (or right click on it and choose merge) You will not see a lot happen when you do it, it's very quick.

If you do rename the file, which has the date in it if you look, make sure you put .reg on the end.

Have a read here before you use the registry section or the advanced as it takes a bit of time to avoid making those 'wrong decisions' we all seem to end up doing sometimes
http://www.internetrotsyourbrain.com/ccleanerbeginnersguide/

Its a personal choice depending on how confident you are in putting things right if there are problems. As for me, I always do a backup, no matter how trivial the change is. CCleaner is so fast, the backups only take a second or two and the files are small since you are just backing up the changes and not the entire registry. I'm no expert, but it seems to me even a pro would find it easier to merge a backup file than manually re-edit if there are problems associated with the registry cleaning.

About once a month I delete the backup files that are over a month old.

I don't personally bother(any more). I use ERUNT which makes a back up of the registry at system startup.