Raspberry Pi connected to TV via HDMI and powered by 5V, 750 mA adapter shows grains on the display and repeatedly reboots after showing the OS installation options. I have connected the Wi-fi dongle and a wireless mouse to the USB ports.
This is almost certainly a power issue. The recommended power supply is at least 1 amp and 2 amps is what is suggested for proper performance. Powering the mouse and WiFi via the Pi, might be possible with a better power supply, however, I would suggest a powered USB hub. A quick search of this site will show many WiFi related problems can be traced to insufficient power.