Newish 149GB HDD being eaten alive and filling up with unknown data..

My laptop PC is an IBM Thinkpad that recently had it's 30GB HDD changed out for a larger 149GB HDD. The original Win XP SP 3 was transferred from the 30GB HDD to the new HDD. Data was previously backed up to my portable 500GB HDD.
Almost at once a report of trojan MURLO was fixed by Spyware. Full deep scans were run and all appeared to be well excxept the Free HDD space soon plummeted from 103GB to 20GB and falling. It appears that all my genuine data is fine but the Free space has been invaded and captured. I have no idea how to solve this and recover the Free space.

Another oddity (to me) is an Unknown device in Device Manager called ROOT\LEGACY_BEEP\XX_NPF_XX

I have run diagnostic tests like Chkdsk (no errors) and Smart Defrag which shows that space is occupied just about all over the HDD with little Free space. I also fully run Norton 360 and Malwarebytes.

Any suggestions gratefully received.

Adrian