You originally stated "Also my comp is Athlon 1.3 GHz, 256Mo ram... hd is not fragmented, good transfer rate.
Now you say 'In fact I have 320 Mo, which is really enough for XP SP1... "
Would be nice to give accurate specifications so proper advise given. I seriously doubt it's a fault with CloneDVD, either a software or hardware issue. Take furballi's advise and download Enditall to stop all startup programs, this should eliminate any software conflicts.