There seems to be a trend (excuse the pun) for internet security suites to demand being the only antimalware you use recently......Norton IS2007 does the same.
However they may have a point about software conflicts,but i don't agree with the view about scanning files at the same time with more than one application.
Lets start with Spybot S&D......the only real time scanner that has is it's Tea Timer registry monitor.Not enabled by default.
Spybot blocks things by inactive registry entries,as does SpywareBlaster (which has no active monitoring).
Spyware Gaurd does use active real time file monitoring and so does windows defender.
I recently removed Defender from my system and noticed a small improvement in speed...replaced it with Spyware Terminator which suites my needs much better and with no performance hit.
You could try removing all the programs mentioned by Trend.Then reinstall your Trend 2007 suite.
The reason for this being rhat one of your real time scanners may have interfered with the original installation.
Then reinstall your additional programs one at a time to see which ,if any is clashing with Trend.
With Spybot don't activate Tea Timer (if you use it) at first.
My gut instinct is the likely culprits are Windows Defender and maybe Spyware Guard.