Can anybody tell me please, what disks to use in my Acer Aspire 3680 ? (rewritable ones)

If the disk you use is a CD-R (Recordable) it can't be erased, as this is a one use Disk. If you used a CD-RW (Rewritable) just put it into your Drive and go to My Computer, rightclick the Disk Drive and choose "Erase this CD".

If you want to see what type of CD your Drive can use (ie, CD+r or CD-R), go to Device Manager (Start> RightClick My Computer> Properties> Hardware Tab> Device Manager) and expand the CD/DVD Drive listing. You should see there what type your computer can read. most modern computers can read both, and even old ones can read CD-R, but you might need to get CD+R if you're unlucky.

Hope that helps.

I believe your laptop will only use DVD+R DVD+RW CD+R or CD+RW

the difference being - or + because there are 2 types of discs. most writes can use both types.