Well you haven't given many clues for us to work on, but there is a few things to check.
1. Although XP (on its own) will run on 128M of RAM, in practice 256 is absolute minimum for running simple apps. I wouldn't attempt to run anything complex (CAD, or two Office type apps) in less than 512M RAM
2. Harddrive space. The OS needs around 2G, plus your apps, plus your data. Even for simple apps you'll need at least 20G empty drive to install and work with XP. I wouldn't consider doing an XP install on less than 60G free.
3. Did you install on a freshly formatted HD, or is this an upgrade? If upgrade, then your registry is almost certainly full of old junk. You will do better to back up all your old data (and check the backup!), then format your drive (if it's big enough, see above), install XP, re-install all your apps, then restore your data.
4. Celeron is generally reckoned to be about half the speed (in true performance terms) of an equivalent number Pentium 4, so you're at the bottom end of the acceptable performance range for simple apps. and considerably under-spec for graphics or CAD usage.
5. Look at the apps you are running. Your system should be OK for email and web browsing, but won't be up to scratch for anything much heavier.
6. Virus/spyware/other malware. If you've been on the internet, or received any emails you may be infected. Make sure you have one good antivirus program running continuously, plus at least one spyware checker running. You should also run a second independent antivirus and malware checker at least once a week, or more if your machine slows down or shows any signs of instability.
7. Google for antivirus and antispyware. I use AVG and EWIDO for frontline use and PANDA as an alternative. For spyware I use Lavasoft Adaware and Spybot, but there are plenty of others.
When you have digested that lot you can come back and let us know what apps you're using and how you get on. Updating old machines can be a very time-consuming and unsatisfactory operation, and with the current cost of new machines is often not commercially viable.
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