If you can, PLEASE upgrade to at least XP Pro, or even better Windows 2000.
If the machine is sold to you with a 300W PSU, then it'll do for the time being. Things to note are that comps don't tend to run much stuff which uses lots of power - apart from the CPU, what really sucks power are really powerful graphics cards and lots of 7200 rpm IDE HDD's (or even a coupla SCSI). I use an Antec TruPower 480W PSU since the less drain I put on it, the cooler and moe reliably it runs. And cos the TruPower series are such kickass PSU's, I could run a Ti4600, 6 15k SCSI HDD's and dual processors offof it, and it wouldn't feel a jot. If anyone else out there is planning on buying a PSU, I can heartily recommend an Antec TruPower (esp the TrueBlue, which is what I have
). I previously ran a 1 GHz P3, two 7200 rpm HDD's, two CD-ROMs and a truckload of fans and case lights off a 250W PSU, and never ran into any problems.
One thng you should all note: very very few PSU's can actually run at their rated wattage (hence why I went for the TruPower, which can).
Your stock CPU cooler will be sh1te, and your CPU will run at 55 degrees idling. Buy and fit a better one.