Same as other said. On a "newer" machine with decent specs, no issues.....well almost.
I had one customer with Vista.....decent processor, 4 gb ram, etc. etc, but wouldn't run Kaspersky without a SIGNIFICANT slow down. Seems on this machine and this machine only, Kaspersky would constantly be pounding the processor and ram. Re-installed newer version of K. (2012-1013), checked for other issues, however same thing! Finally gave up, put on Eset and it ran fine.
I've put on other similar machines with no issues.
??????? one of life's mysteries, I guess.
Kaspersky and Eset are my go to anti-virus for paid options.
Once tried on a P4 with Win XP, 1 GB ram, 2 point something processor (Celeron, I believe)..............real dog SLOOOOOOW!