I would like to install windows 7 and office 2007...is possible?
Barely, HP says the system is capable of up to 1 GB of DDR Memory, so that would leave you with netbook style performance.
If anything a graphics card upgrade to a Nvidia GT or AMD Radeon would help with the compatibility.
I suggest saving money and buying a new computer, as these are too old to run windows 7 without having missing drivers.
Pavilion a210n Memory Specifications
Standard Memory 256 MB (removable)
Maximum Memory 1.0 GB
Memory Expansion 2 sockets
Memory Comments PC2100 DDR SDRAM DIMMs
CPU Type 2.5GHz Intel Celeron
Model Comments 400MHz FSB, Intel 845GL Chip
Most information that's available for Socket 478 PCs is from when those motherboards were manufactured and when the largest RAM modules were 512MB, so 2 sockets summed up only 1GB... but the largest DDR modules made are at least 2GB and possibly up to 4GB (found no info yet to verify 4GB sticks were ever made)
I own a Dell Dimension 4500 with the D845 Chipset that according to the Corsair RAM configurator, can use 2GB and according to Astra32 (System Information application) the total capacity is 3GB (3072MB).. I've also read some users that claimed to be running 4GB on a Socket 478 computer, so it's more than likely your computer can handle at least 2GB of RAM, and more than that would have to be analized by a RAM configurator considering their information is most likely outdated, or with a System Information Application.