it's perfectly safe. This is from the Eraser Help File: (after explaining about the Free Space Wipe)
If you are running Windows NT or 2000 and the file system on the drive is NTFS, Eraser will next overwrite the free space on the Master File Table (MFT). The reason why this is done is that on NTFS file system, clusters are not necessarily allocated for files smaller than the size of a MFT record, but the file is stored completely in the MFT (the file is then said to be resident). If you have insecurely deleted such a small file, the free space on the MFT still may contain the file body and therefore, it must be erased as well. Windows 9x does not support NTFS file system so this step will be skipped.
Finally, the names of all previously deleted (or erased) files will be overwritten. On FAT{12,16,32} partitions this is done by going through all directory entries and overwriting deleted file entries. On NTFS partitions (Windows NT and 2000 only), Eraser creates maximum length files until the unused entries in the Master File Table are overwritten.
Although this refers to Windows NT and 2000 it includes XP and Vista.