Constraining CPU usage

Hi Guys!
  
  It seems i cannot get a clear answer wherther it is possible to limit a CPU 
usage by a user process and should i do that at all.
  
  I just wana run "tar -xzvf /usr/backup/pub.tar.gz /usr/home/pub" and do not 
cause something like this:
  
      PID   USERNAME PRI NICE    SIZE    RES STATE  TIME WCPU  CPU 
      3993  backup          139     20  8672K  8068K  RUN     19:56 87.65% 
87.65%
      load averages:  0.99,  0.98,  0.88  
  
  As you can see in the figure above, renicing wont work. 
  
  Any hints? What to read? Thank you.
  
  
  Nash
  
  FreeBSD 5.3-RELEASE
  Timecounter "i8254" frequency 1193182 Hz quality 0
  CPU: AMD Duron(tm) processor (1202.73-MHz 686-class CPU)
  

                
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