Win XP Locking Up

I have an old PC in a building outside the house which is running Win XP Pro, it has a celeron 1.7 processor(I think) and I have increased the RAM from 512 Mb to 1Gig. Everything works fine as a stand alone but now I've added a wifi card (there's no telephone line to the building and it's too much trouble putting one in) to get online, basic internet is OK, email and single searches etc but if I use more than 1 internet window at a time or email and search at the same time the damn thing locks up. Needs to be hard rebooted.
I've tried my laptop in the building, which is a 1.8 processor with 1Gb RAM and Vista, and it's perfectly fine so there's a problem with the desktop somewhere. I've stripped the whole thing down and rebuilt it reloading Win XP and all the drivers and still the same. Tried using different browsers, IE, Mozilla and AN Other I can't remember the name of and still the same.
Anybody have any ideas?

Could be nearly anything Chriss....Think all you can do is try to eliminate things 1 at a time...
Have you opened up the Windows task manager & seeing how much of the CPU & memory you are using?? Is it maxing out for some reason? Are you showing all the memory as available.
Second thing is the new wireless card...where did you get the driver for it from? Is the driver actually compatible with XP, or is it designed for Vista/7??
You could try using 1 of the free registry cleaners (even the one on CC Cleaner, although that is not the best) as maybe you have an issue there...

It is hard to guess what else it could be....the fact that you added memory & a wi-fi card suggests that the issue is probably there, but, equally, it could be that putting the extra load on the pc has highlighted an issue that has been there a long time...
The laptop I had before this one only had 256mb Ram & a 1.5 AMD processor running XP. It was slow & did very occasionally (like maybe once every 2 months) lock up completely, but that was with multiple tabs (5 or 6 including mail & sometimes Messenger) open.

Thanks Bob,

Have looked at the task manager and CPU usage only goes up to 70 something percent, 500 or so on memory all mem is showing available.
Checked compatibility of the wifi driver before I bought the card (ebay) and cross checked with the maunfacturer (Atheros) website, all good for XP.
Done all the bells and whistles re cleaning the registry etc, ran all my virus, malware and spyware programs, although I don't think its that.
As I say I stripped the PC down to nothing cleaned the hard drive out completely and rebuilt reloaded OS (which is a legit copy) and all that goes with it, its a really bare PC with just basics.
I removed the new memory card and replaced with the original and the same thing happens, meaning I went from 1 Gb down to 512Mb.
So it all points at the wifi card or setup. Have searched for max data transfer details for the wireless but can't find any....but I'm guessing that would only cause the connection to the router to drop or slow.. so checked the reports from the router to the PC, all looks good and comparable with the laptop. Theres always a good or excellent signal between the router and PC..
Now I'm lost for where to look next.

Might be worth while looking at a site like this...
http://www.computertooslow.com/speed-up-computer.asp

They offer to test your pc via software - I imagine that they will come up with all sorts of issues & then tell you that there is a charge to fix them.... But, even if you just do the free testing bit maybe it will give you some clues as to where the problem might lie..
You could take it into the house & connect it directly to the modem rather than via wi-fi. If it was no different then that is a fair indication that the card is not the issue.
As you have already eliminated the memory from the problem it is at least crossing things off the list..

I am not sure if this is much different to CC Clean depends how much you believe the advertising blurb that goes with it. But the standard version is available for free, so it might be worth you trying.
Advanced SystemCare Free 3.7.3
http://download.cnet.com/Advanced-Syste ... 07614.html

will give them a go Bob thanks. Need to go an buy a LAN cable to connect straight to the router, should be in towntomorrow will drop by a computer shop.

Not sure if this is the problem but older Celeron processor did not have much of their own memory so uses part of your installed RAM and with the wifi working and demanding processor memory it may overwork the processor causing system to lock up. Just my thoughts, may not be problem

OK put the XP running PC into the house and connected direct to the router via LAN cable all OK no lock ups. Horrah, problem is with the wifi.
AND, low and behold if I looked closer to start with I might just have seen a good possibility for the problem, the wifi on the laptop runs at 100Mbps and the wifi on the PC runs at 54Mbps.
So solution, run a phone line into the outside building and relocate the router or upgrade the wifi card...
Guess it's time to search good old eBay again.
Thanks for your comments all.