If it's set up to really play hardball, using all its options all the time, then it may affect level loading and stuff significantly when large files have to be read from disk. On my work system, VSE 8.0i can stall my A64 3000+ for 45 seconds at 100% CPU usage, when I try to open the Help file in Microsoft Access 2000 (it's a .CHM file). It would stall it for longer than that if 45 seconds weren't the "give-it-up" timeout period we're using.
If you want something lighter, then AntiVir might be good if you don't mind doing the updates manually.