LoL I haven't heard of an android virus yet... Maybe it's some kind of crap app.
Anyway, I've used CM7.2 before, but battery life was horrible. And none of the custom kernels would work correctly with it, except it's own.
Now I'm using Hybrom with Rage kernel, and the batt life is phenomenal compared to CM.
(direct link for st15i) After download, place it on your SD card. (It's a .zip file)
You have to have an unlocked bootloader. For that you can use the link at the top of the guide.
If you're unlocked, you have to flash Rage kernel for st15i http://dl.dropbox.com/u/31230492/rage_kernel_mini.zip
To flash it, connect your phone in fastboot (connect USB to PC while holding UP volume button). The LED should be blue for fastboot, green for flashmode.
open FlashTool, click the thunder icon, select fastboot mode, then select kernel to flash (boot.img).
After it's flashed, turn the phone on. When you see the sony ericsson logo, keep pressing volume DOWN button until you get to recovery.
When you'r there, select factory reset, clear cache, go to advanced and clear dalvik also. Then go select zip file from SD card, and select hybrom (or any other ROM for that matter).
That's all there's to it i think