I would definitely Scan you puter for spyware. Sounds to me like you have a spyware/malware program running in the back gournd. Bye the way. you can always shut down the puter by unpluging it or turning off the PSU via the PSU on/off switch.
but leaving it on is no problem...just make sure the monitor is protected wiht screen saver or monitor hybernation!!!
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.