BT/McAfee anti virus.
McAfee won't help either. There's no way to exclude the Radarbox folders in McAfee, so all file activity in RB will be virus-checked :(
When BT switched to McAfee (from Norton), I found Radarbox performance suffered. So I switched to Microsoft Security Essentials instead, and excluded the Radarbox folder from virus checking.
For info, I run Vista Home Premium on a Dell Dimension Intel Dual Core 2.8ghz with 2gb RAM. I install Radarbox on a separate drive, formatted as FAT32 to avoid Vista's terrible Transactional NTFS, which also hits I/O performance.