It depends on your RAM, HDD speeds, and CPU speed. 256MB 5GIG 2GHz isnt going to do well. dual cores should be better at it. But it is safe, but will slow down the computer. The OS matters to. Windows 98 & SE willrestart Defragmenter if you do anything while trying to surf the web,because the hard drive writes/reads when doing that and alters the info, making the defrager have to restart.