There are a number of IT Malls: Panthip, Fortune, Zeer/Rangsit where you will find retail PCs from all the major manufacturers. Fortune is easier to get to if using public transport, and the shops there are branches of those in Panthip. Many people here have PC's built-to-order, I'd recommend that as it is quite easy to do.

It is nigh on impossible to recommend a DSL provider as much is dependent on your location. Do you have a fixed line? TOT or True? You can get DSL from the fixed-line provider or from another ISP. For example, I have DSL (1,024/512: 1,070 THB/month) from CSLoxInfo on a TOT fixed line (107 THB/month). If you have a fixed line number then you can look up service availability. Check out www.adslthailand.com for links. My service has been fast, as advertised, and reliable.

By far the best approach is to check with neighbors to see what they are using and if they like it. I'm not 100% certain but you may need to have a work permit to get service? My service got turned up in 72 hours even though the provider said it would take 10 days.