Computer Lockup

Start with some maintainence, erase off the temp files in the temp directory, erase *.tmp in windows, reboot, run scan disk, reboot, run defrag, If you've done all this without freezing up then freeze up is not a problem, run an antivirus package. Run ad-aware to clean up spyware. If it is still locking up, open case and blow out fans and heatsinks with a can of compressed air. leave cover off and start back up to see if heat is the issue.(if it doesn't lock up its probably a heat issue). If it still locks up it may be time to reload the operating system.
If that doesn't fix then fdisk/format/reload. Unless you can narrow down something in particular for locking it up. I'ld start all this process without the modem installed. If it only acts up with the modem installed it's either a n interupt problem between modem and serial ports or the modem card is bad. keep up updated with what your trying, what operating system is it? How old is the pc? Maybe the battery is wearing out. Good luck