I have used Cubase for over a year now and it has crashed twice on that time (on one of those two times due my own error). So I think the blame goes somewhere else than the software.
Windows XP & Cubase is a stable combination. There have to be either hardware problem or just non-updated, spywared and overburdened Windows.
Try this:
1. Make backups for all important files such as Cubase projects, mp3's, etc. Also visit
www.zonelabs.com and
www.free-av.com and add ZoneAlarm and Free-AV to the backup CD/DVD. Edit: And think twice what you need to back up because better safe than sorry.
2. Format whole hard disk. Spare nothing.
3. Disconnect the internet cable that the computer doesn't connec to the internet until you want it to.
4. Install Windows XP.
5. Install ZoneAlarm and FreeAV.
6. Connect the internet cable and run Windows Update. Get all updates. Then get latest drivers for your hardware.
7. Install needed programs, like Cubase, Winamp, etc. Don't forget the updates.
If you still have problems your big bucks are gone to things that are not-so-much-worth-of-big-bucks.
Also get Ad-Aware to get rid of various spyware stuff that gets on your system from various places. Also a popup killer is a must.
Edit: You could also make two Windows installs each on sepperate drives. That way you could strip down the other XP and make it as audio OS and use the other one for general use. That way everything works fine.
It's a good habit to do totally clean OS install from time to time. Lots of useless software and files are gathered on the HD in due time and this slows down the whole system.
If you have spare CD's or DVD's you could burn the OS with the installed software on the CD's or DVD after step 6 so that next time you make the clean isntall you can format everything and just copy the backupimages from the CD's/DVD so that you don't have to start updating everything again. Much faster that way.