I just bought a new hard drive as my last one gave up (suddenly) and i have installed winxp, but as i was installing programs i ran out of disc space, and have discovered that the drive must have been partitioned and i have mistakenly installed windows on a 2gig partition and left the other 78gig free. Is there any way i can use the rest of the drive without installing everything again.
Not really - 2gb is too little to run XP on. You could get some third party partitioning software such as Partition Magic but to be honest I think you would be better off wiping clean and starting afresh.
It's of little use to you with a 2gb partition so when installing XP choose to repartition - either into 2 x40gb or leave as one partition. If you search these pages you will find a lot of debate about the value of partitioning; I fall into the camp that advocates partitioning for ease of backup and organisation.
Don't know why it came like that unless there was a partition with drivers etc - I once got a PC that had such a feature.