I don't recommend parallels. It runs really slowly. I recommend you use bootcamp instead.
It depends on the strength of the system you run it on. If you have an older dual core machine with 2-4GB of ram and a 5400rpm hard disk of course Parallels isn't going to run great. If you have adequate hardware Parallels runs great in my experience. Even on my 2009 13" MBP with 8GB of ram and an 128GB SSD Parallels can run a Windows XP VM with 2.75GB of ram dedicated to it surprisingly well and is quite smooth. My 2010 6-core 3.3ghz Xeon Mac Pro with 24GB of ram, dual SSDs (raid 0) and dual 600 Velociraptors (raid 0) runs 64-bit Windows 7 VMs faster than many dedicated PCs and laptops I've seen and I can game in that VM quite well, if I choose.
...it all depends on the hardware how well Parallels runs.