Does it affect anything if you multitask (windows 7 x64) while running SMART tests to scan for HDD errors?
It should not, excluding the 'offline tests' smart operations. Even with those I'd be surprised if you saw anything more significant than a small performance difference. Most SMART operations only involve the drive firmware and not the platters where your data is stored.
When you say "running SMART tests" some clarification needs to be made.
From this wiki:
S.M.A.R.T. is a series of built-in self tests that are run and monitored by the hard drive. Viewing these results can be done at any time whether using the hard drive or not. There is really no "testing" done, but rather a viewing of these already monitored tests.
However if you do any diagnostics on the drive then there is a possibility that something might happen. My recommendation is to avoid testing your hard drive while doing any sort of tasks. Also be sure to make backups of the data. It's better to be safe than sorry.