But why no just get a legal copy of Windows 10?
Just a heads up that Windows 10 can be installed on that computer and when asked for a key there is an option to state that you do not have a key. Windows 10 will install and you can then see if Windows 10 is a good match for your hardware which it should be given your system has 4GB DDR2. Then at a later point you can pay to register it if Windows 10 runs at a good pace on that system vs paying money now to upgrade to 10 and then find out that its not a good pairing for your processing needs and speed.
Looking up your computer specs I found this for your model which might be a match:
Athlon 64 X2 3800+ (2.00 GHz)
1 GB DDR2
200 GB HDD
Windows XP Media Center
No Screen
NVIDIA GeForce 6100
That CPU can run Windows 10 64-bit and make full use of the 4GB RAM. Unless your XP is 64 bit you have only been using 3GB of 4GB.
Only area of concern for this computer is the GPU, if your using the integrated Geforce 6100 that is a very weak GPU in todays terms. My daughter was watching Netflix and Hulu for a while on a Geforce 6200 AGP 256MB and there was some lag. If you add a low cost video card to this system like a Radeon HD5450 1GB Video card for around $25 it will play Netflix without any problems. My daughters computer is a Athlon64 X2 4450B 2.3Ghz and it has a Radeon HD 5450 PCI Express 1GB Video Card.
If you already upgraded the GPU and not running integrated video, then you might be fine with the card you have. My wife about 6 months ago was using netflix on a Geforce 9800GT 512MB Video Card and it was fine for watching Netflix, Hulu, and Crackle.