When you say "new PC" that's a warning flag. A lot of off-the-shelf PCs come loaded with bloatware (i.e., trial versions of Norton utilities), have unneeded drivers, and load lots of unnecessary routines at startup which eat memory. Memory utilization of 50% while recording vocals with a single stereo backing track seems way out of line, even with 4 GB of RAM (which I would consider to be the minimum requirement these days for a DAW).
Often these kinds of computers, particularly laptops, lead to "SONAR is doing strange things" threads where eventually, the community finds the source of the problem and slims the computer down. If you're comfortable with Device Manager, try disabling drivers for things you don't need (particularly the one mentioned in
Tip 35 in my Tip of the Week thread). Also check out the startup programs in msconfig and uncheck the ones you don't need. That should help get the optimization process started. Finally, make sure you have the latest
v2.7 drivers for the Lexicon Omega.
What is your latency setting? Are you using ASIO?